SLLA602 March   2024 LM5110 , LM5111 , TPS2811 , TPS2811-Q1 , TPS2812 , TPS2813 , TPS2814 , TPS2815 , UCC27323 , UCC27324 , UCC27324-Q1 , UCC27325 , UCC27423 , UCC27423-EP , UCC27423-Q1 , UCC27424 , UCC27424-EP , UCC27424-Q1 , UCC27425 , UCC27425-Q1 , UCC27444 , UCC27444-Q1 , UCC27523 , UCC27524 , UCC27524A , UCC27524A-Q1 , UCC27524A1-Q1 , UCC27525 , UCC27526 , UCC27527 , UCC27528 , UCC27528-Q1 , UCC27624 , UCC27624-Q1 , UCC37323 , UCC37324 , UCC37325 , UCD7201

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1Introduction
  5. 2How a Gate Drive Transformer Works
  6. 3Benefits of a Gate Drive Transformer
  7. 4Design Considerations of a Gate Drive Transformer
    1. 4.1 Duty Cycle Limitation
    2. 4.2 Transients and Noise
    3. 4.3 Calculations
    4. 4.4 Power Loss Calculations
    5. 4.5 Bias Supply Thermal Calculation
  8. 5Summary
  9. 6References

4.5 Bias Supply Thermal Calculation

Another common use for gate drive transformers is gate drive bias generation. By connecting a full-bridge rectifier on the secondary side, a gate drive transformer can be used to generate a floating DC voltage. This floating bias can be used to power isolated gate drivers.

GUID-20240226-SS0I-SK9S-QCTM-RJ1S2PDMMSK2-low.svg Figure 4-3 Example Schematic of a Push-Pull Bias Supply Using the UCC27444 Gate Driver IC

Gate driver ICs are designed to drive the gates of power switches, which is a capacitive load. In a bias supply, the gate driver is providing power to a bias supply, which is a mostly resistive load for the gate driver IC. Because of this difference in loads, the power dissipation equations in the gate driver IC data sheets do not apply to bias supply circuits.

Calculating the power dissipation in this application is straightforward. Assuming a 1:1 turns ratio, the load current from the bias supply appears as a DC current in the gate drive IC. Therefore, we can use the following equation as an estimate:

Equation 18. P = I l o a d 2 × R o h + R o l

To improve the accuracy of the equation, we can add the RMS current due to magnetizing current in the primary:

Equation 19. P = R o h + R o l × I l o a d 2 + V D D × t o n L m a g × 2 × 3 2

As an example, suppose that instead of driving a half-bridge as shown in Figure 2-3, we instead used the same setup to generate two 12V, 3W bias supplies. Neglecting magnetizing current, we can estimate the power dissipation as shown:

Equation 20. P = 3 W 12 V × 2 2 × 5 + 0.6 = 1.4 W

This estimate uses the typical values for Roh and Rol from the UCC27624 data sheet, and ignores the magnetizing current factor because magnetizing current is determined largely by the transformer. In this case, we can expect to dissipate about 1.4W in the driver output stage. Multiplying by RθJA to get a rough estimate of heating, we can expect the D package to self-heat significantly, as the RθJA is 126.4ºC/W. The DGN package, which has a much lower RθJA of 48.9ºC/W, self-heats less. Thermal performance also depends on other parameters such as board layout and copper thickness, but RθJA is designed for package to package comparisons in the same condition.

Another option can be to use a driver such as UCC27444. The UCC27444 driver uses a PMOS only pullup structure. The PMOS only structure results in a lower Roh to DC current compared to the hybrid structure driver UCC27624. The typical Roh of UCC27444 is about 1.2Ω. By re-calculating Equation 20 with the UCC27444 parameters, we can expect to dissipate about 0.475W. In this case, UCC27444 is estimated to dissipate about one-third of the amount of power UCC27624 dissipates.