This article will take an in-depth look at DC DC power supplies.
The article will look at topics such as:
This introductory section delves into the essential concepts of DC-DC power supplies, focusing on their design, construction, and basic operating principles.
A DC-DC power supply is an electrical device designed to convert power from one set of voltage and current specifications to another, enabling efficient voltage level transformation within electrical systems with minimal energy loss.
Functionally similar to transformers but for direct current (DC), a DC-DC converter changes input voltage levels while maintaining impedance. The power supply itself does not create power; its output relies entirely on the input energy. Although some energy is lost as heat within the circuit, modern DC-DC power units attain efficiencies as high as 90%, while earlier models usually range between 80% and 85%. These devices can handle a diverse spectrum of voltages, from low (as found in small batteries) to high (used in power transmission).
Recent advancements in integrated circuits allow these power supplies to function at higher switching frequencies. Designers can now minimize the size of essential magnetic components, such as output filter inductors and transformers, within switching power supplies. Higher frequencies facilitate rapid recharging of the output capacitor, reducing the need for multiple capacitors while achieving swift and accurate dynamic output. The advent of wide bandgap semiconductors, like silicon carbide (SiC) and GaN transistors, supports these higher frequencies with lower losses, enabling greater power densities over conventional silicon MOSFETs.
A wide array of DC-DC converters populate the market, including buck, boost, non-inverting buck-boost, and flyback models.
These topologies are often employed based on specific application requirements, balancing performance, size, and cost considerations.
| Selecting External Components Based on Properties | ||||||||
|---|---|---|---|---|---|---|---|---|
| Coil | CL* | SD* | External TR. | External TR. (Bipolar) |
||||
| L | DCR | (Power Mosfet) | RB | CB | ||||
| For Higher 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 Response | Small | - | Large | - | - | - | - | |
To identify the ideal inductance value, refer to the provided chart, which features values based on oscillating frequency and output current or load. The correlation between oscillating frequency and inductance significantly influences the choice. Opt for a coil with minimal DC current resistance if possible. Decreasing the inductance value increases the coil’s maximum current (IPEAK), thus achieving the highest current output for a particular inductance value.
As inductance rises, maximum current losses at the switching transistor subside, enhancing efficiency. However, this can lead to increased losses from the coil's DC current resistance, reducing efficiency. When choosing a coil, focus on the rated permissible current. Exceeding this limit may result in heat generation in the inductance coil, magnetic saturation, and efficiency loss. Extended currents past permissible values can severely damage the IC.
| Inductance Value Selection | |||||
|---|---|---|---|---|---|
| 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 |
Opt for a diode with a lower forward voltage to diminish losses from voltage drops during forward conduction and maximize efficiency. This also aids in lowering the startup voltage in step-up circuits. Ideally, the diode's forward voltage should not exceed 0.6V at peak coil current. The terminal capacitance must be minimal; higher capacitance can decelerate switching speeds and escalate spike noise when the diode switches on or off, leading to raised switching losses.
When choosing a diode, ensure it has low reverse leakage current to minimize spike noise and retain efficiency, especially under low load conditions. The reverse leakage current can swell with temperature or in diodes that exhibit low forward voltage (VF) and high current capacities.
In step-up power supplies, the diode's current rating must be two to three times higher than the peak coil current at the lowest input voltage. For step-down supplies, this rating must be two to three times higher than the peak coil current at the maximum input voltage. Be considerate that with Pulse Frequency Modulation (PFM) control, the max current value may decrease. Caution is advised to adapt. The diode should be rated for a voltage at least 1.5 times the output voltage in step-up supplies and 1.5 times the input voltage in step-down supplies. This ensures reliable operation under all conditions.
Understanding temperature characteristics of ceramic load capacitors is vital. Units without B properties may diminish capacitance due to temperature shifts or DC bias effects. This can impair performance; therefore, testing when using tantalum, aluminum electrolytic capacitors, or OS-CONs to guarantee compatibility in operation is recommended.
For applications like these, use tantalum capacitors with 10µF capacitance. If the output exceeds 100 milliamps, ensure a tantalum capacitor with at least 100µF capacity is installed. Load capacitor’s equivalent series resistance (ESR) should be maintained between 0.1Ω and 0.5Ω. Should you use low ESR capacitors like OS-CON, ensure sufficient phase compensation to avoid abnormal operation. Even when utilizing OS-CONs, tantalum, or aluminum electrolytic capacitors, accurate compatibility with the IC must be ascertained.
For aluminum electrolytic capacitors, load capacitance should be twice to thrice higher than values recommended in common circuits, to avoid deficits under low temperatures or ESR spikes. Pairing a tantalum capacitor above 10µF or a ceramic one from 0.1µF to 1.0µF in series can diminish ESR. The critical factor here is the allowed ripple current; excessive levels might overheat the IC, shorting its longevity. Capacitance with ripple below 50mV is preferable.
For step-down DC-DC power sources, the input capacitor acts as the ripple reduction unit and should be situated as near as possible to the IC for optimal ripple filtering.
For step-up DC-DC converters, connect the input capacitor directly to the IC, reducing impedance effects of the power supply on the system. As long as capacitors feature low ESR values, the type used can be flexible, unlike load capacitors.
When input voltages dip under 1.2V, the gate voltage may not suffice for a Power MOSFET. In such cases, a bipolar transistor is advisable. For elevated outputs, choose Power MOSFETs with low ON resistance. Bipolar transistors, due to their lower gains (hFE), and increased base current, tend to be less efficient compared to MOSFETs in substantial current applications.
Power MOSFETs with lesser input and output capacitance (<1000pF) and accelerated switching speed are recommended for enhancing efficiency, thanks to shorter switch-on/off delays and faster rise times.
Ensure the MOSFET's source-to-gate cutoff voltage is lower than the input. For supplies under 1.2V, bipolar transistors are preferred. When driving ICs, input voltage must be sufficiently higher than the MOSFET's cutoff voltage. While low ON resistance is favorable, it often means increased capacitive loads. For peak currents, step-up supplies should use MOSFETs rated for two to three times the peak current. Conversely, for step-down supplies, rating should double the output, multiplied by the step-down ratio. Analyzing current waveforms and MOSFET temperatures before finalizing is crucial. Monitor peaks as they increase in PFM control. Step-up supply MOSFETs should exceed 1.5 times the output, and step-down supply should be at least 1.5 times the input voltage. Verify actual components to confirm pin voltages align within rated specifications.
If the MOSFET must handle all losses resiliently, select one rated for power dissipation beyond expected loss levels. Ensure an ample safety margin when dealing with high-output voltage and current. Examine heat dissipation within the operational temperature spectrum, incorporating extra cooling if needed.
Choose bipolar transistors with current gain (hFE) between 100 and 500. High gain transistors offer low base currents but may display increased leakage when off.
Whenever feasible, employ bipolar MOSFETs with improved switching speeds, as better switching speeds enhance efficiency. Ideal capacitance ranges at the collector output to around 10pF. Parameters like swift turn-on times, reduced fall, and storage should characterize effective MOSFET switching speed.
Select base resistance between 250Ω and 2kΩ. Lower values, under 250Ω, can impair IC operation. Smaller resistors (200Ω-500Ω) enhance current output but may reduce light-load efficiency. Conversely, higher resistors (700Ω-2kΩ) curb current output but boost light-load efficiency.
The speed-up capacitor (CB) improves efficiency. Its value remains stable regardless of base resistance or oscillation regulator. Boosting its value significantly enhances switching speed and current capacity. Even modest increases show considerable changes in switching speed.
DC-DC power supplies are vital in providing a steady, precise current essential for developing reliable electronics. They transform DC voltage to desired levels using circuits with inductors, diodes, and power transistors. Voltage smoothing and regulation at output follow the conversion. Increasingly used in portable electronics, their rise replaced AC conversion as the primary means of elevating DC voltage before power semiconductors revolutionized the field.
Several factors must be considered when choosing a DC-DC power supply:
Understanding the system's load output power demand, including output voltage and input current, is crucial. Output voltage tolerances vary based on load current, input voltage, and ambient temperature. Load current specifications must state maximum, minimum, and typical values.
Dissimilar to AC/DC sources, DC-DC power supplies lack input voltage standardization. Clarifying the input voltage range is essential when selecting a DC-DC converter.
DC-DC power supplies split into isolated and non-isolated setups. Isolated versions internally employ transformers for input-output electrical separation. Buck converters with transformers, known as forward converters, transfer energy while the primary side conducts. Similarly, boost converters with transformers are termed flyback converters, wherein magnetic components store energy when the primary switch is on and deliver it when off. Industry-standard isolated converters, commonly branded as power bricks, abound with manufacturers offering them widely.
Much like AC/DC models, most DC-DC supplies deliver regulated voltages. Unregulated outputs suffice in scenarios where they are tolerable, often in lower-power converters.
DC-DC power supplies appear in various mounting styles and packages. For applications requiring direct printed circuit board (PCB) mounting, surface and through-hole mounting, along with singular or dual inline pins, are available. Chassis-mounted options cover applications necessitating that type. Many supplies conform to DIN rail mounting for industrial usage. Both encapsulated and open-frame variations span an array of mounting and packaging choices.
Most electronics adhere to EMC (Electromagnetic Compatibility) and EMI (Electromagnetic Interference) guidelines to ensure device non-interference and resistance to external electrical noise. While individual circuit certification is rare, DC-DC power supplies are sometimes certified for these standards, usually at a system-wide level.
In parallel to EMC and EMI regulations, safety standards are often mandatory for electronic equipment, typically applied to the finished device. Though seldom certified independently, safety certification ensures power supplies protect users from dangerous voltages when necessary.
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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