Switching Power Supply Design for High Voltage and CurrentBy ZM Peterson • May 4, 2020
From integrated circuits to large power supplies from discrete components, your next PCB will need some sort of power regulation scheme in order to operate properly. We like to think that power sources provide smooth AC or DC output at all times, but this is almost never the case. Precision analog systems and digital systems need stable, predictable voltage output with high efficiency.
With this in mind, what determines the efficiency, stability, and power output in switching power supply design? We can boil this down to five areas:
- Switching converter topology
- Supporting circuitry
- Component selection
- Switching frequency
- PDN impedance
The last two points above are usually afterthoughts in switching power supply design, but they are the most important for low-level systems, such as low power IoT devices and precision analog systems. Here’s what you need to know about switching power supply design.
Switching Power Supply Design Tips
Low-Level DC Systems
A typical switching power supply for low power/low-level digital systems may contain control circuits in a small IC package. In this case, your main concern is ensuring your unregulated input remains within the correct range. For battery-powered systems, the battery voltage will drop as the battery discharges, so you need to ensure that the output will remain at the desired voltage/current to keep the system running. A typical topology is to place an LDO regulator at the output stage, which will ensure consistent output voltage and current as long as its input voltage is above the required headroom. You generally need to place input and output EMI filter circuits, and the inductor and capacitor required for regulating output power. Read this article to learn more about the various DC-DC converter topologies you can use, as well as how the output relates to the duty cycle and output ripple.
High Power Systems
For high voltage/low current, or for low voltage/high current, there are switching regulator ICs you can purchase that will include the regulator circuitry you need. In this case, you need to follow the same strategy for your layout and component selection as you would when working at low power. Switching regulator ICs are available that provide a range of output powers and can accept a broad range of inputs.
For high power systems (high voltage and high current), the situation is quite different. You’ll need to layout each functional block in your switching power supply design from scratch. You generally will need to consider the following design aspects to make the system produce the desired power output:
- PWM generator. This sets the output for a buck, boost, buck-boost, or other converter topology to a specific level, depending on the duty cycle. In modern regulator ICs, PWM generator might be programmable and be integrated into the converter. In other cases, you can supply the PWM signal with an MCU or a separate generator IC.
- Control circuitry with feedback. Control circuits generally rely on feedback for accurate control, and switching power supply design is no different. High power systems normally use a current sense amplifier to check that the current output is at the desired level. The output amplifier is then used by a PWM generator or MCU to adjust the output voltage by adjusting the duty cycle of the PWM signal.
- Rugged components. The last thing you need is for your power system to fail because your components could not handle the current/voltage they need to supply. Semiconductors (specifically FETs used in switching power supplies) can fail if overdriven to extreme levels (thermal failure).
- Thermal management. Even a 99% efficient power regulator will reach a high temperature if heat is not dissipated from the system. Keeping the system cool generally requires heat sinks, fans, or both.
Switching power supply reference design from Maxim Integrated. Notice the separated driver IC, MOSFET, and passives on the board.
If you’re designing DC power conversion with an AC power source, it’s best to include a power factor correction (PFC) circuit for the AC mains. This will ensure that the switching regulator stage in your power supply draws a nearly sinusoidal source of current, rather than drawing current in short bursts. This increases overall power factor of the entire regulator, which in turn reduces the amount of power lost as heat (i.e., higher efficiency).
Choosing a PWM Switching Frequency
The switching frequency of the PWM signal in your switching power supply will determine the level of losses as this signal is responsible for modulating the gate voltage in the driving MOSFET. Using a higher frequency causes the MOSFET switch on and off more often, which then allows less to accumulate in the MOSFET. However, the edge rate is also critical as it determines whether the MOSFET channel is sufficiently modulated into the OFF state. With a slow edge rate, the MOSFET can remain conducting even though the PWM signal may have dropped to 0 V.
By using a faster edge rate, you can push the MOSFET deeper into the OFF state, which then reduces heating losses in the switching regulator section. Combining a higher PWM frequency and faster PWM edge rate allows physically smaller components to be used in the regulator circuit. However, the tradeoff is greater conducted and radiated EMI as a PWM signal will emit at higher frequencies. ~100 kHz PWM frequencies are typical in most power supplies, but a highly efficient switching power supply design could be made more efficient and use smaller components when the PWM frequency is brought to 1 MHz with ~1 ns edge rate.
Setting the PWM switching above the rolloff frequency for your switching regulator will prevent switching noise from conducting to the regulator output. The rolloff frequency is defined in the circuit diagram for the basic buck-boost converter shown below. Note that you can use a larger PWM switching frequency if you can use smaller components in your switching regulator. You can read more about this in one of my recent articles on Altium’s PCB Design Blog.
Buck-boost switching power supply design with rolloff frequency equation.
Isolation and PDN Impedance
One point we haven’t discussed explicitly is isolation in your switching power supply design. Power isolation is a great way to add a safety measure to your power system. This portion of power supply design, as well as incorporating control feedback in an isolated system, is extensive enough for its own article.
To learn more about PDN impedance and its effects on digital and analog systems, you can read more from some other articles on the NWES blog:
- Analog vs. Digital PDN Impedance: How to Design for Each Signal
- Causes of Ripple Voltage: Power Supply Noise and Transients in Your PDN
- PDN Design and Power Rail Ripple Suppression in Your PCB
For your layout, be sure to follow the IPC-2221 and IPC-2158 standards to ensure your traces do not reach excessively high temperature, and to prevent ESD between exposed conductors. These tips just scratch the surface of power supply design, but the right design firm can help you construct a compliant layout that is manufacturable at scale.
At NWES, we’ve created low power digital and analog systems, and we’ve built high power DC systems with different switching power supply design topologies. We know how to create a high quality, fully manufacturable PCB layout for your system. We're here to help electronics companies design modern PCBs and create cutting-edge technology. We've also partnered directly with EDA companies and advanced PCB manufacturers, and we'll make sure your next layout is fully manufacturable at scale. Contact NWES for a consultation.