Determining the Voltage Rating of MOSFETs for DC-DC Applications

 

A half-bridge MOSFET configuration typically used in DC-DC converters. Here, M1 and M2 form the key switching elements driven by a controller. In any DC-DC converter design, selecting MOSFETs with the correct voltage rating is crucial to ensure reliable and efficient operation. This blog will dive into the considerations necessary for determining the appropriate voltage rating for MOSFETs in DC-DC applications.

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1. Understanding the Application and Voltage Requirements

The image illustrates a half-bridge topology in a DC-DC application, with an input voltage $V_{DD}$ of 12V and a current of 5A flowing to the load. To choose the proper voltage rating for the MOSFETs (both M1 and M2), several factors must be considered, such as the input voltage, switching node behavior, and potential transients or spikes during switching events.

In general, the MOSFET voltage rating should be higher than the maximum voltage the MOSFET will experience during operation, including any voltage spikes or transients.

2. Key Considerations for MOSFET Voltage Rating

Several critical points influence the choice of MOSFET voltage rating in DC-DC converters:

a) Input Voltage

For a DC-DC converter, the MOSFETs will be exposed to at least the input voltage $V_{in}$. In this case, with $V_{DD} = 12V$, this serves as a baseline. The MOSFET voltage rating must exceed this value to accommodate typical operation and avoid breakdown.

b) Switching Node (Drain to Source Voltage)

In a half-bridge configuration, the drain-source voltage (Vds) of the MOSFETs can swing significantly during switching. For the high-side MOSFET (M2), the voltage across the MOSFET can vary between the input voltage (12V) and ground (0V), depending on its switching state. The low-side MOSFET (M1) also experiences high drain-source voltage when M2 is on, and vice versa.

Thus, each MOSFET will experience the full input voltage across its drain-source terminals during operation. However, if there are transients or overshoot, the voltage can exceed this.

c) Voltage Spikes (Overshoot and Ringing)

During the switching transitions of the MOSFETs, parasitic inductances in the PCB layout, especially from the traces and package, can cause voltage spikes (known as ringing). These spikes can add significant voltage stress to the MOSFET beyond the nominal operating voltage.

A typical rule of thumb is to expect spikes that are 20-30% higher than the input voltage. For instance, with a 12V input, transients may push the MOSFET voltage to about 15-16V or even higher, depending on the layout and snubber design.

d) Safety Margin

A conservative safety margin is crucial to ensure reliable operation over temperature, component aging, and varying loads. Common practice is to add a 20-30% margin above the highest expected voltage. For example, if the input voltage is 12V and transients could reach 16V, a MOSFET with a voltage rating of 20V would be sufficient.

However, in many designs, designers opt for standard voltage ratings of 30V or 40V MOSFETs for DC-DC converters with 12V inputs, which provides ample margin for safety.

3. Calculating the MOSFET Voltage Rating

Let's break this down step by step with the given circuit example:

Step 1: Determine Input Voltage

The input voltage is $V_{DD} = 12V$.

Step 2: Estimate Switching Node Transients

Assume the switching transients could introduce a voltage spike of up to 30%. This would mean the maximum drain-source voltage seen by the MOSFETs could be:

$$V_{spike} = V_{DD} \times (1 + 0.3) = 12V \times 1.3 = 15.6V$$

Step 3: Add a Safety Margin

Adding a 30% safety margin to handle unexpected voltage variations and ensure long-term reliability, the MOSFET voltage rating should be:

$$V_{MOSFET} = V_{spike} \times (1 + 0.3) = 15.6V \times 1.3 \approx 20V$$

Therefore, a MOSFET with at least a 20V rating would technically work, but designers often choose 30V or 40V MOSFETs for additional robustness.

4. Choosing the Right MOSFET

Based on the above calculations, the following guidelines apply when selecting MOSFETs:

• Low-Side MOSFET (M1): The low-side MOSFET sees the full input voltage when the high-side MOSFET is conducting. Therefore, it must be rated to handle at least the peak voltage, including any spikes.

• High-Side MOSFET (M2): Similarly, the high-side MOSFET will be exposed to the full input voltage plus any transients. A similar voltage rating is required for this MOSFET as well.

5. Common MOSFET Voltage Ratings in DC-DC Converters

For a 12V input system, as shown in the example, commonly used voltage ratings are:

• 30V MOSFETs: These are widely used in low-voltage applications like 12V automotive systems, where input voltages and transients remain below this threshold.

• 40V MOSFETs: Often selected for higher safety margins and to ensure reliable operation, even under worse-case transient conditions.

• 60V MOSFETs: These may be chosen in systems where the input voltage could fluctuate to higher levels (e.g., in automotive designs with cold crank or load dump scenarios).

6. Thermal and Switching Performance

While the focus here has been on voltage rating, the choice of MOSFET should also take into account other critical parameters:

• Rds(on): Low Rds(on) minimizes conduction losses, which is crucial for improving efficiency, especially at high currents (5A in your circuit).

• Gate Charge (Qg): A low gate charge is beneficial for reducing switching losses and ensuring the MOSFET can switch rapidly at higher frequencies.

• Thermal Considerations: Power dissipation from both conduction and switching losses should be analyzed to avoid overheating. MOSFET packages with good thermal dissipation characteristics (like D2PAK or PowerPAK) are typically preferred in high-power applications.

Conclusion

For a DC-DC converter with a 12V input, the MOSFET voltage rating should account for the input voltage, switching transients, and a safety margin. In this case, 30V or 40V MOSFETs are ideal for ensuring reliable operation. Selecting the proper MOSFET also involves considering other parameters like Rds(on) and thermal performance to balance efficiency and robustness.