This article will take an in-depth look at DC DC power supplies.
The article will look at topics such as:
This chapter will cover the fundamentals of DC-DC power supplies, including their design, construction, and operational principles.
A DC-DC power supply is a device that transforms electrical energy from one voltage and current level to another. It allows for efficient conversion between different voltage levels in electrical systems with minimal power loss.
A DC-DC converter serves a similar function to transformers but for direct current. It adjusts the input voltage to a different level while maintaining similar impedance. It is important to note that the power supply itself does not generate power; all output energy is derived from the input. While some energy is lost as heat due to inefficiencies in the circuit, modern DC-DC power supplies can achieve efficiencies of up to 90%. Older models typically have efficiencies between 80% and 85%. These supplies can handle a wide range of voltage levels, from very low (like those in small batteries) to very high (used in power transmission systems).
The advancement of integrated circuits with enhanced features has made it possible to operate power supplies at higher switching frequencies. This progress allows designers to reduce the size of magnetic components, such as transformers and output filter inductors, in switching power supplies. Higher switching frequencies quickly replenish the output capacitor’s charge, enabling the use of smaller and fewer capacitors while achieving fast and precise dynamic output performance. The introduction of wide bandgap semiconductors like silicon carbide (SiC) and GaN transistors supports higher switching frequencies with lower switching losses, allowing power supply designers to attain greater power densities compared to traditional silicon MOSFETs.
Various types of DC-DC converters are available on the market, including buck, boost, non-inverting buck-boost, and flyback converters.
These four topologies are the most commonly used. Selecting the appropriate topology involves balancing size, performance, and cost based on the specific requirements of the application.
| Choosing External Components By Properties | ||||||||
|---|---|---|---|---|---|---|---|---|
| Coil | CL* | SD* | External TR. | External TR. (Bipolar) |
||||
| L | DCR | (Power Mosfet) | RB | CB | ||||
| For Heigher Current | Small | Small | Large | Small VF |
(Low ON Resistance) | Small | Large | |
| For Higher Efficiency | Light Load | Large | Small | - | Small IR | - | Large | Small |
| Heavy Load | Large | Small | - | Small VF | (Low ON Resistance) | Small | Large | |
| For Low Ripple Output Voltage | Large | - | Large | - | - | - | - | |
| For Better Transient Resonse | Small | - | Large | - | - | - | - | |
When selecting the inductance value, you can refer to the chart below, which outlines values based on oscillating frequency and current output or load. Since oscillating frequency and inductance value are proportionally related, this relationship plays a crucial role in determining the appropriate inductance. It's advisable to use a coil with low DC current resistance whenever possible. As the inductance value decreases, the coil’s maximum current (IPEAK) increases, leading to a maximum total current output for a given inductance value.
As the inductance value increases, the switching transistor losses due to maximum current are reduced, leading to higher efficiency at a specific inductance value. However, with an increase in inductance, the losses from the coil's DC current resistance also rise, which can lower efficiency. When selecting a coil, the rated permissible current value is a critical parameter to consider. If the current exceeds this permissible value, the inductance coil will generate heat, leading to magnetic saturation and a drop in efficiency. If the maximum current surpasses the allowable current value, it could severely damage the IC.
| Choosing Inductance Value | |||||
|---|---|---|---|---|---|
| Load | 50kHz | 100kHz | 180kHz | 300kHz | 500kHz |
| Light Load | 330μH | 220μH | 100μH | 47μH | 22μH |
| Medium Load | 220μH | 100μH | 47μH | 22μH | 10μH |
| Heavy Load | 100μH | 47μH | 22μH | 10μH | 6.8μH |
Using a diode with a low forward voltage is recommended to minimize losses due to the forward voltage drop and improve efficiency. In step-up circuits, this also helps reduce the working start voltage. Ideally, the forward voltage should be less than 0.6V at the coil’s maximum current.
It's important for the diode's terminal capacitance to be low. High capacitance can slow down the switching speed and increase spike noise during the diode's turn-on and turn-off times. Reduced switching speed can lead to higher switching losses.
Choose a diode with a low reverse leakage current to minimize spike noise and maintain efficiency, especially under light load conditions. High reverse leakage current can increase with temperature and with diodes that have low forward voltage (VF) and high current ratings.
For step-up DC-DC power supplies, the diode's current rating should be two to three times higher than the maximum coil current at the minimum input voltage. For step-down DC-DC power supplies, the rating should be two to three times higher than the maximum coil current at the maximum input voltage. When using Pulse Frequency Modulation (PFM) control, be aware that the maximum current value will decrease, so caution is advised.
The diode's voltage rating should be at least 1.5 times the output voltage for step-up DC-DC power supplies and 1.5 times the input voltage for step-down DC-DC power supplies. In practice, the diode's rated voltage should exceed the maximum terminal voltage to ensure reliable operation.
When using ceramic load capacitors, it's important to understand their temperature characteristics. Capacitors that do not have B properties may experience reduced capacitance due to changes in ambient temperature or DC bias effects, potentially affecting their performance. Ensure that tantalum capacitors, aluminum electrolytic capacitors, and OS-CONs are properly evaluated for their operational suitability.
For tantalum capacitors, select a capacitor with a capacitance of around 10µF. For applications requiring a current output greater than 100 milliamps, use a tantalum capacitor with at least 100µF capacitance. The equivalent series resistance (ESR) of the load capacitor should be between 0.1Ω and 0.5Ω. If using a low ESR capacitor like an OS-CON, ensure adequate phase compensation, as inadequate compensation can lead to atypical operation. Ceramic capacitors are generally unsuitable for these applications. Even when using OS-CON, tantalum, or aluminum electrolytic capacitors, confirm their compatibility with the IC and verify their performance thoroughly.
When using aluminum electrolytic capacitors, ensure the load capacitance is two to three times greater than the value recommended in general application circuits to prevent capacitance loss at low temperatures and increases in equivalent series resistance (ESR). In such cases, connect a tantalum capacitor of 10µF or more, or a ceramic capacitor with a range of 0.1µF to 1.0µF in series to reduce ESR. The key parameter for aluminum electrolytic capacitors is the permissible ripple current. Excessive ripple voltage can generate heat and shorten the lifespan of the IC. It is advisable to select a capacitor with a ripple voltage output lower than 50mV.
In step-down DC-DC power supplies, the input capacitor serves as the ripple reduction capacitor for the IC. It should be placed as close to the IC as possible to effectively filter out power supply ripple.
The input capacitor must be connected directly to the step-up DC-DC power supply IC to minimize the input power supply's impedance effects on the IC. Unlike load capacitors, the choice of capacitor type can be flexible as long as the capacitor has a low equivalent series resistance (ESR).
When the input voltage is below 1.2V, there may be insufficient gate voltage to fully turn on a Power MOSFET. In such cases, a bipolar transistor should be used instead. For higher output currents, choose a Power MOSFET with low ON resistance. Bipolar transistors generally have a lower current gain (hFE), which means that for high current applications, the base current increases, leading to reduced efficiency compared to a MOSFET.
Select a power MOSFET with output and input capacitances lower than 1000pF. The MOSFET should also have a high switching speed to improve efficiency. A higher switching speed is characterized by shorter switch-on delays, rise times, and switch-off delays.
Choose a MOSFET with a source-to-gate cutoff voltage significantly lower than the input voltage. For voltage supplies below 1.2V, a bipolar transistor is more suitable. When powering ICs in step-up DC-DC power supplies, ensure that the input voltage is higher than the MOSFET's source-to-gate cutoff voltage.
The ON resistance between the source and drain of the Power MOSFET should be low. However, MOSFETs with low ON resistance typically have larger output and input capacitances. There is a trade-off between these capacitances and ON resistance that must be considered.
For step-up DC-DC power supplies, select a MOSFET with a current rating two to three times higher than the peak current. For step-down DC-DC power supplies, the rated current should be about twice the current output multiplied by the step-down ratio. Ensure to check the current waveform and MOSFET temperature before finalizing the choice. Pay close attention to current peaks, which often increase in Pulse Frequency Modulation (PFM) control.
The MOSFET's rated voltage for step-up DC-DC power supplies should be at least 1.5 times the output voltage. For step-down DC-DC power supplies, the rated voltage should be 1.5 times the input voltage. Always verify actual components to ensure that the voltage between pins does not exceed the rated voltage.
If the MOSFET must handle all the losses in the circuit while maintaining efficiency, it should be rated for power dissipation higher than the expected losses. For high output voltage and current, ensure the MOSFET can handle the power loss with an adequate safety margin. Verify the heat dissipation within the operating temperature range and consider additional cooling if necessary.
Choose a current gain (hFE) in the range of 100 to 500. Bipolar transistors with very high current gain tend to have lower base currents but may exhibit higher off-state leakage currents.
Utilize bipolar MOSFETs with high switching speeds whenever possible, as increased switching speed improves efficiency. Select a MOSFET with an output collector capacitance of approximately 10pF. Ensure that the MOSFET has high switching speed characteristics, including short turn-on time, fall time, and storage time.
The base resistance should be chosen within the range of 250Ω to 2kΩ. Resistance values lower than 250Ω can affect the operation of the IC. A smaller base resistance, around 200Ω to 500Ω, increases current output but reduces efficiency at light loads. Conversely, a higher base resistance, close to 700Ω to 2kΩ, decreases current output but improves efficiency at light loads.
The speed-up capacitor (CB) is used to enhance efficiency. The value of this capacitor is unaffected by the base resistance and switching regulator oscillation. Increasing the speed-up capacitor value significantly improves switching speed and current supply. Even a small increase in the capacitor value will result in noticeable changes in switching speed.
DC DC power supplies provide a constant and precise current, which is crucial in developing mission-critical electronic components. A DC DC power supply converts DC voltage to the appropriate DC voltage level using a circuit consisting of inductors, diodes, and power transistors. After conversion, the voltage is smoothed by specific capacitors and regulated at the output. These power supplies are increasingly used in portable electronics like mobile phones and laptops. Prior to the development of power semiconductors, the primary method for increasing DC voltage was through AC conversion.
Several factors should be considered when selecting a DC DC power supply:
One of the primary considerations in choosing a DC DC power supply is determining the load output power needs of the system. This includes the input current and output voltage. The output voltage may have tolerance ratings depending on conditions such as load current, input voltage, and ambient temperature. Specifications for load current should include maximum, minimum, and typical values.
DC DC power supplies differ from AC/DC power supplies in that the input voltage is not standardized. It is essential to specify the input voltage range when selecting a DC DC converter.
DC-DC power supplies can be categorized into non-isolated and isolated configurations. An isolated converter uses a transformer within the powertrain to provide electrical separation between the input and output. In this setup, buck converters with transformers are known as forward converters, as they transfer energy during the time the primary side is conducting.
On the other hand, a boost converter equipped with a transformer is referred to as a flyback converter. In this configuration, energy is stored in the magnetic components when the primary switch is on and is then transferred to the secondary side when the primary switch turns off. Many power supply manufacturers offer these isolated converters, commonly referred to as power bricks in the industry.
DC DC power supplies typically provide a strongly regulated voltage output, similar to AC/DC power supplies. Unregulated voltage outputs may be used in applications where this is acceptable, often in lower-power converters.
DC DC power supplies come in various mounting styles and packages. For applications requiring direct PCB mounting, options include surface mounting, through-hole mounting, and different pin configurations such as single in-line or dual in-line pins. Chassis-mounted power supplies are available for applications requiring that style. Many power supplies are designed for DIN rail mounting in industrial settings. Both encapsulated and open-frame power supplies are offered in a range of mounting and packaging options.
Most electronic equipment must meet EMC (Electromagnetic Compatibility) and EMI (Electromagnetic Interference) regulations. These regulations ensure that equipment does not interfere with other devices and that external electrical noise does not disrupt its operation. DC DC power supplies may be certified to comply with these regulations, although the entire system is often certified, and internal circuits might not require individual certification.
Like EMC and EMI regulations, electronic equipment must also meet safety regulation requirements. Safety certifications are usually applied to the end equipment. Internal components may not need certification but can be certified if necessary. Safety certification is essential for power supplies that isolate equipment operators from hazardous voltages.
A power supply typically features a switching system that divides a constant power source into manageable increments of energy. This is followed by a filtering system that reconstructs these increments into a stable, usable output power source.
At its core, a non-isolated power supply comprises four fundamental components: a switching transistor, a diode, an inductor, and an output capacitor. Various topologies can be created with these components and the appropriate control circuitry to meet specific performance goals for output voltage and efficiency.
In numerous applications, power is distributed at higher voltage levels to achieve optimal system efficiency. The role of the power converter in these scenarios is to reduce the incoming voltage to the lower level required by the load. Power converters that produce an output voltage lower than the input voltage are known as Buck Converters.
Essentially, a switched DC-DC power supply or regulator is a type of circuitry that uses an inductor, a power switch, a capacitor, and a diode to transfer power from the input to the output. These systems can be categorized in various ways to achieve different types of power conversion.
When the switch (SW) is closed or ON, energy is transferred from the input voltage (Vin) to the load. By adjusting the ON and OFF times of the switch, the amount of energy delivered to the output can be controlled. The duty cycle of a power converter, which ranges from 0 to 1, is the ratio of the switch ON time to the total period (ON time + OFF time). By varying the SW ON time, the output voltage of the power converter can be adjusted from 0 V to nearly Vin.
In a buck converter, the inductor is connected directly to the output capacitor and load. Even when the switch is open, current can continue to flow through the diode to the output via the antiparallel diode. When used as a regulated output converter, the power supply controller adjusts the duty cycle to maintain a constant output voltage, despite changes in Vin or system parameters. Non-isolated buck converters can achieve very high efficiencies.
When the chopper is switched ON, the source is directly connected to the load, resulting in the output voltage (VO) being equal to the input voltage (Vs). The duration for which the chopper remains ON is known as the ON time, represented by TON.
During the chopper's ON time, current through the load increases exponentially and reaches its peak by the end of TON. If TON is sufficiently short, the current will not reach a steady state, allowing the increase in current to be approximated as linear.
Boost converters utilize similar circuit components but arrange them differently to achieve varied performance outcomes. These converters elevate the input voltage to deliver an output that exceeds the initial voltage. In a boost converter, the energy is transferred to the output in a two-phase sequence, with an inductor serving as a temporary storage device. When the switch is engaged, current flows into the inductor, and the longer the switch remains ON, the more magnetic energy is accumulated.
The diode prevents the current from reversing from the load, while the output capacitor maintains the output voltage. Once the switch is disengaged, the magnetic energy stored in the inductor is directed through the diode, which recharges the capacitor. This process replenishes the energy supplied to the load when the switch was in the ON position. This dual-phase operation can lead to high peak currents in the components, which may affect the efficiency in higher power applications. Consequently, this design is generally suited for low power uses.
When CH turns OFF, the current through the inductor L doesn’t drop immediately but instead diminishes gradually. This causes the current to flow through the diode D and the load during the OFF period. The emf generated in L is reversed, leading to the load voltage being the sum of the inductor emf and the source voltage. Hence, the output voltage is increased, expressed as VO = VS + L(di/dt).
Buck-boost converters are a fusion of buck and boost power converter technologies. They integrate a buck converter on the input side and a boost converter on the output side, utilizing a common inductor. This configuration allows the converter to maintain a stable output voltage regardless of whether the input voltage is above or below the desired level. Consequently, the output voltage remains consistent as the battery transitions from full charge to full discharge.
Such converters are especially useful in battery-operated devices where maintaining a stable output voltage is crucial, even as the battery depletes. This design extends the operational life of the device and ensures consistent performance as the battery’s charge wanes.
When the input voltage exceeds the output voltage (for instance, 14.5V from a fully charged battery in a 12V system), the converter operates in buck mode with the boost switch open and the output diode continuously conducting. Conversely, if the input voltage drops below the set output level (as the battery discharges), the input switch remains ON, and the converter functions as a boost converter to sustain the output voltage. A single control system manages voltage regulation and switches between buck and boost modes as needed.
According to the equation provided, the output voltage always has an opposite polarity compared to the input. Hence, a buck-boost circuit is often referred to as a voltage inverter.
Cuk converters can be viewed as a combination of buck and boost converters, featuring a single switching component and a shared capacitor to transfer energy between the stages.
Similar to the buck-boost circuit with an inverting topology, non-isolated Cuk converters usually produce an inverted output voltage, which can be either higher or lower than the input voltage. While inductors are commonly used as the main energy storage element in most DC-DC converters, Cuk converters rely primarily on capacitors for energy storage.
A charge pump is a DC-DC converter that uses capacitors to store and transfer charge for voltage boosting or reduction. These circuits can achieve high efficiency, often between 90% and 95%, while maintaining a simple design.
Transformer-based isolated DC-DC power supplies primarily come in two configurations: flyback and forward. In both cases, the transformer provides isolation between the input and output.
The flyback type operates similarly to a buck-boost converter but uses a transformer for energy storage.
In a forward converter, the transformer transfers power from the primary side to the secondary side through direct coupling when the switch is closed.
In DC-DC converters, diodes are typically used for output rectification. However, conventional power diodes, like freewheeling or rectifying diodes, can suffer from high conduction losses, especially at elevated currents where the forward voltage drop (VF) is around 0.6 to 0.7 volts.
To reduce these losses, power MOSFETs are employed instead of traditional diodes. This approach significantly improves efficiency compared to standard or Schottky diodes. Although synchronously driving power MOSFETs with the main switches involves complex control and additional components, the efficiency gains make this method widely adopted in various switching power supplies.
DC-DC power supplies are essential for many electronic devices, as most require DC power. Portable gadgets like laptops and smartphones rely on batteries for energy, but these devices often need different voltage levels for their various circuits. Larger DC-DC converters are also used to maximize power from photovoltaic systems and to charge batteries, with some models capable of adjusting both voltage and current, such as those used for powering LEDs.
DC-DC power supplies are commonly used in portable electronics like laptops and smartphones, which primarily draw energy from batteries. These devices contain multiple circuits, each requiring specific voltage levels that may differ from the battery or external power source voltage.
As a battery discharges and its voltage decreases, switched DC-DC converters can boost the voltage from a partially depleted battery, saving space compared to using multiple batteries to achieve the same result.
Most DC-DC converters regulate output voltage, though there are exceptions. For example, some high-efficiency LED power supplies focus on regulating current for LEDs, while simple charge pumps might only double or triple the output voltage.
In renewable energy applications, DC-DC power supplies, known as power optimizers, manage energy harvested from wind turbines and photovoltaic (PV) systems. Traditional transformers used for voltage conversion at mains frequencies (50 to 60 Hz) become bulky and expensive for higher power levels, often resulting in energy losses due to winding resistance and eddy currents in the core.
In contrast, high-frequency DC-DC converters that use inductors or transformers are much smaller, lighter, and more cost-effective. These converters are often used even in situations where a main transformer would be needed. For instance, in domestic electronics, it's common to rectify the mains voltage to DC, then use switch-mode techniques to convert it to high-frequency AC at the desired voltage before rectifying it back to DC.
The intricate circuitry of DC-DC power supplies is often more cost-effective and efficient compared to simple mains transformer circuits with the same output. These power supplies are widely used in DC microgrid applications, accommodating various voltage levels.
DC-DC power supplies have several key specifications:
Efficiency refers to the proportion of input power that is delivered to the load. Many DC-DC power supplies achieve efficiencies of 90% or higher. When selecting a DC-DC power supply, ensure that the energy source can compensate for any inefficiencies. A practical approach is to assume an 80% efficiency for the power supply and provide 125% of the load power. For example, for a 4W load, use a 4W DC-DC power supply with a 5W energy source. Efficiency is often represented in curves, showing maximum efficiency at specific load currents, and can decrease at lower output powers due to the power required to operate the circuitry.
This rating indicates the maximum peak current that the DC-DC power supply can provide to the load. While the power supply may be capable of delivering more current, doing so may cause excessive heat and potential failure.
This specification denotes the highest ambient temperature at which the DC-DC power supply can operate reliably under full load. Exceeding this temperature can lead to overheating, damage, or automatic shutdown as a protective measure.
Ripple voltage refers to the fluctuations in the output voltage. The ripple voltage rating of a buck converter should meet the specific requirements of the application to ensure stable performance.
This metric reflects how well the output voltage is maintained despite variations in load current and input voltage. A power supply with a 1% regulation rating will maintain the output voltage within 1% of the nominal value across the specified ranges of load and input voltage.
DC-DC power supplies use feedback loops to regulate output. Changes in input voltage or load current may cause temporary deviations. The speed of the control loop response indicates how quickly the power supply can adapt to these changes and stabilize the output voltage.
The voltage rating defines the range within which a DC-DC power supply can effectively step up or step down the input voltage.
DC-DC power supplies can operate at very high frequencies, allowing for miniaturization. However, increasing frequency can also raise other mechanism losses, resulting in a trade-off between efficiency and size.
DC-DC power supplies come in various mounting styles and packages. Options include surface mount, through-hole mount, and pin configurations like single in-line or dual in-line. Chassis mounting and DIN rail setups are available for industrial uses. Encapsulated and open-frame designs offer several mounting and packaging options.
Electronic devices must comply with Electromagnetic Compatibility (EMC) and Electromagnetic Interference (EMI) standards to ensure they do not disrupt other equipment and that they remain functional despite external electrical noise. DC-DC power supplies may be certified for these standards, though often the entire system is certified rather than individual internal circuits.
Similar to EMC and EMI standards, electronic equipment must adhere to safety regulations. While safety certifications are generally required for end equipment, not for internal components, they can be obtained if necessary. Safety certification is crucial for DC-DC power supplies used to isolate operators from hazardous voltages.
DC/DC power supplies otherwise known as DC/DC converters are power supplies that convert a DC voltage of a certain magnitude to one of a different magnitude to supply a device. They are quite important these days since most electronic devices require DC power for Industrial, Medical, and Telecom markets. They are classified into isolated and non-isolated converter topologies, which are chosen based on the needs of an application.
Isolated power supplies are driven by the Telecom industry, which uses a negative (-48V) bus system for long term reliability. Advanced wireless systems have become a major player in the communications industry. The quest for higher efficiency and power density has been driven by the adoption of smaller and higher performing non-isolated converters.
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