DC-DC Converter PCB Layout Design Guidelines

A DC-DC converter is a type of switching power supply that operates based on high-frequency switching actions to convert input voltage into the desired output voltage. This process utilizes components like capacitors and inductors to store and transfer electrical energy, ensuring the stability and quality of the output voltage. Compared to linear power supplies, DC-DC converters have the advantage of their switching transistors operating in saturation and cutoff states, which reduces heat generation and improves efficiency. However, one potential issue with switch-mode power supplies is that their output DC voltage may have significant ripple, and the operation of switching transistors can introduce spikes and pulse interference.

dc-dc converter

With this, TechSparks concludes its introduction to DC-DC converters. If your work in electronic circuit design involves the use of DC-DC converters, then this article will be of assistance. 

Table of Contents

Multilayer Circuit Design​

In DC-DC converter circuits, high-frequency and high-current signals are quite common, and the result can be potential interference with other signals. To address this, it is recommended to construct a circuit design with at least four layers and include a well-placed ground plane for effective noise absorption.

A practical approach to building multi-layer circuits is to position the DC ground layer or DC power layer between the high-power component layer and the sensitive signal routing layer. This shields it from interference caused by high-noise power traces and power components.

The table below illustrates the layout of 4-layer and 6-layer circuit structures, where (A) and (C) are considered unsuitable layer structures for switch-mode power supplies. You can see that the small-signal layer is sandwiched between the high-current power layer and the ground layer, which increases the capacitance noise coupling between the high-current voltage power layer and the analog small-signal layer. On the other hand, (B) and (D) represent an effective design that helps reduce coupling noise between different layers. In this design, the ground layer is used to shield the small-signal layer.

(A)(B)
Layer: 1 Power ComponentLayer: 1 Power Component
Layer: 2 Small SignalLayer: 2 Ground Plane
Layer: 3 Ground PlaneLayer: 3 Small Signal
Layer: 4 DC Voltage or Dround PlaneLayer: 4 Small Signal
Layer: 5 Small SignalLayer: 5 DC Voltage or Dround Plane
Layer: 6 Power Component/ControllerLayer: 6 Power Component/Controller
(C)(D)
Layer: 1 Power ComponentLayer: 1 Power Component
Layer: 2 Small SignalLayer: 2 Ground Plane
Layer: 3 Ground PlaneLayer: 3 Small Signal
Layer: 4 Small Signal/ControllerLayer: 4 Small Signal/Controller

Here are recommended PCB layout techniques for DC-DC converters:

  • When dealing with transient currents in DC-DC circuits, design the traces or routing to be as wide as possible and keep them as short as possible.
  • Buffer circuits are often used to attenuate or smooth signal fluctuations. To minimize the length of signal transmission paths, place them near the switching transistors and output rectifier diodes.
  • Avoid parallel lines that are too long, use vertical cross-wiring when conditions permit, and try to use smooth curves at corners to connect wires instead of abrupt corners.
  • Implement reverse current routing to mitigate the impact of magnetic fields on the DC-DC circuit.
  • Separate analog ground and power ground to prevent interference between analog and digital signals. Connect them near the power source to ensure a common reference point for the entire system.
  • Keep input and output lines of filters separate; do not bundle input and output lines of the switching power supply.
  • If necessary, place output filter inductors in the ground return path.
  • Isolate areas where pulse currents flow from input and output terminals.
  • Use copper foil for low inductance and low resistance wiring.

EMC Design

DC-DC converters use PWM (Pulse Width Modulation) technology to control the output voltage. In this circuit, pulse signals are generated at a certain frequency, and the width of the pulses is adjusted according to the desired output voltage. Typically, the pulse waveform is rectangular, which means the circuit contains a significant amount of harmonic components, resulting in potential interference issues.

Power EMI Filter

In the case of DC-DC converter circuits, sources of interference typically concentrate in two main areas: one is the switching circuit itself, and the other is the output rectification circuit. An effective measure is to incorporate a power EMI (Electromagnetic Interference) filter into the design.

Think of it as a “filter” for electronic devices, allowing the smooth passage of 50Hz or 400Hz AC power while reducing or suppressing the propagation of interference signals. This not only prevents interference from entering the device but also avoids its spread into the power network, effectively addressing power interference issues at their source.

When selecting a power EMI filter, several parameters need consideration, such as rated voltage, rated current, leakage current, insulation resistance, and temperature, among others. These parameters should be chosen based on your specific requirements, as different applications may demand different specifications. Finally, install the chosen filter near the power cord socket.

Sealed Shielding

For mitigating radiation interference in DC-DC converter PCB design, TechSparks recommends employing sealed shielding technology. This involves designing the structure of the enclosure in such a way that different parts of the enclosure can make tight contact. This sealed shielding design helps maintain the continuity of the electromagnetic environment inside the device, reducing the leakage or interference of electromagnetic radiation, ensuring the normal operation of the device, and preventing interference with external devices or the environment.

In today’s era where lightweight portable devices are the mainstream, it is suggested to use aluminum alloy for constructing the enclosure to reduce weight. However, one needs to be aware of an issue: compared to other metals, aluminum alloy has poorer magnetic conductivity, which can affect the shielding effectiveness. To overcome this issue, a layer of nickel can be plated on the surface of the aluminum alloy enclosure or a layer of conductive paint can be applied to enhance the enclosure’s conductivity. Additionally, a high permeability material can be applied to the inner surface of the enclosure to help absorb or guide the electromagnetic field. For enclosure connection parts, conductive adhesive bonding or continuous welding seam structures can be used. If the enclosure needs to be disassembled, it is recommended to use conductive rubber strips for compression to ensure signal continuity.

Suppressing Output Noise

Different strategies for suppressing output noise in DC-DC converters can be formulated based on various frequency ranges:

Below 10kHz: In this frequency range, the noise is considered normal noise. Addressing the converter’s issues in this range can be achieved by appropriately placing LC filters within the circuit.

From 10kHz to 10MHz: Within this frequency range, common-mode noise is more significant, often caused by switching power supplies or other high-frequency components. To tackle this issue, it is recommended to employ common-mode suppression filters. These filters consist of inductors and capacitors and use high-permeability, well-frequency-response ferrite magnetic materials. Typically, the inductance values range from 1 to 2mH, and capacitor values range from 3300pF to 4700pF.

Above 10MHz: This frequency range falls into the high-frequency category, where traditional filter designs may no longer be effective. Instead, improvements to the filter’s configuration are required to efficiently filter out high-frequency noise. Common enhancement measures include multi-stage filters, high-frequency-responsive ferrite magnetic materials, and adjustments to inductance and capacitor parameters.

Heat Dissipation Design

The impact of heat on device reliability is self-evident. When the circuit temperature reaches its limit, components face a risk of failure. For every 2°C increase in component temperature beyond this limit, the reliability of the DC-DC circuit decreases by 10%. When the temperature rise reaches 50°C, the device’s lifespan is reduced to 1/6 compared to a temperature rise of 25°C. The fundamental principle of DC-DC converter thermal design is “source reduction and flow enhancement,” aiming to reduce heat generation at its source and improve heat dissipation.

Reducing Heat Generation

An effective strategy is to employ technologies like phase control or synchronous rectification to enhance the efficiency of the power supply and electronic devices. Phase control adjusts the conduction angle of the voltage to control the time current passes through the circuit, thereby regulating output power. However, this technology may lead to energy wastage due to truncation of certain parts of the voltage waveform. Hence, it is recommended to use synchronous rectification technology in DC-DC converters. In traditional circuits, alternating current is converted to direct current using diodes, resulting in voltage drops and energy loss. Synchronous rectification technology replaces diodes with controllable switching devices like MOSFETs to pass current more efficiently and reduce losses.

Another approach is to minimize the use of high-power components. Typically, heat sources in circuits stem from components, and high-power components generate more heat. Therefore, during design, opt for lower-power alternatives. When multiple heat-generating components exist in a DC-DC circuit, consider merging their functions to reduce the number of these components and, consequently, reduce heat sources.

Enhancing Heat Dissipation

Heat dissipation devices, such as heat sinks, liquid cooling, thermoelectric cooling, and heat pipes, help dissipate heat from the device.

The most common example is heat sinks, made of metal, which efficiently dissipate heat by transferring it to the surrounding air.

Liquid cooling systems, using water or oil, absorb heat generated by components and transport it to a cooling system for circulation and dissipation.

Thermoelectric cooling involves adding a thermoelectric module to the circuit, composed of a series of thermocouples. When current passes through these thermocouples, one side becomes hot, while the other becomes cold, effectively reducing the device or system’s temperature.

Heat pipes consist of sealed metal pipes filled with a special working fluid. When in operation, the fluid evaporates due to heat, transforms into vapor, flows to the cooling area, condenses back into liquid, and releases heat.

Among these methods, heat sinks are often the optimal solution. Forcing air cooling can achieve over ten times the effectiveness of natural cooling but increases project costs and introduces noise and vibration issues. In DC-DC converter circuit layouts, heat sinks are placed on the leeward side or on top of the printed circuit board.

Heat sinks are treated with an oxidation blackening process to increase emissivity. Black paint application is not allowed, as it can affect heat dissipation. Applying three-proof paint after spraying may impact heat dissipation, so some allowance should be provided. The contact surfaces of heat sink installation devices should be smooth and flat, with silicon grease applied to enhance thermal conductivity. Transformers and inductors should use thicker wires to suppress temperature rise.

Safe Design

When it comes to the safety of power circuits and DC-DC converters, it is not only about the proper functioning of the product but also about the well-being of users, particularly concerning potential threats like electric shock and burns.

One critical aspect of ensuring the safety of electronic devices involves leakage current, which refers to the current that may leak to ground or other locations within electrical or electronic equipment. In commercial applications, leakage current is typically required to be between 0.5mA and 5mA. In military or other applications with high safety demands, the requirement is that leakage current must be less than 5mA. However, these values may vary depending on different countries, regions, and applications.

Generally, the magnitude of leakage current is influenced by the capacitance size in EMI filters. Larger capacitors are more effective at storing charge and allowing low-frequency signals to pass through. However, from a circuit design safety perspective, capacitor size should be kept relatively low to prevent potential breakdown during transient voltage spikes in the power grid. While this situation may not threaten personal safety, it can result in the filter losing its filtering function. Engineers, therefore, need to strike a balance between these two considerations.

To mitigate the potential threats of leakage current, a common design principle for plugs and sockets is to make the product end (non-power end) with pins and the grid end (power end) with sockets. Additionally, in the design of plugs and sockets, exposed areas or parts in contact with the human body should not exceed 60°C when the ambient temperature is 25°C. Moreover, the maximum temperature for panels and manually adjustable parts should be maintained below 50°C.

Plug socket design

Capacitor Selection

When alternating current is rectified, it needs filter capacitors to smooth the output voltage. This requires capacitors with a sufficiently large capacitance, making aluminum electrolytic capacitors a good choice. However, one needs to consider the relationship between temperature and lifespan. In general, typical capacitors have a lifespan of around 1000 hours at lower ambient temperatures, and after this time frame, circuit specifications may no longer be met. In temperature-sensitive DC-DC PCB circuits, it’s advisable to opt for capacitors with longer lifespans, and typically, larger capacitors have longer lifespans.

Since DC-DC switch-mode converters obtain energy from the power source in pulse form, significant high-frequency currents flow through the filter capacitors. When the equivalent series resistance (ESR) of electrolytic capacitors is high, it results in considerable losses, leading to capacitor heating. Low-ESR electrolytic capacitors can significantly reduce the heating generated by ripple currents, especially at high frequencies.

Electrolytic capacitors used for rectifying the output of switch-mode power supplies need to have low impedance in the 300kHz to 500kHz frequency range. At this point, the ESR of electrolytic capacitors is low and effectively filters out high-frequency ripple and voltage spikes in switch-mode power supplies. Ordinary electrolytic capacitors start to exhibit rising impedance beyond 100kHz, making them less effective for filtering in switch-mode power supply applications. Therefore, high-frequency electrolytic capacitors excel in such cases.

Since aluminum electrolytic capacitors don’t perform well in the high-frequency range, they need to be combined with ceramic or film capacitors with good high-frequency characteristics.

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