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

4.3 Calculations

Pulse transformer selection is based on two main requirements; the minimum volt-seconds (VS) and voltage drop (droop). The volt-seconds specification is related to the saturation of the transformer. Voltage drop on the primary is relevant because the drop can result in increased conduction losses in the power stage.

The VS is the product between the voltage across the transformer primary, and the duration of that voltage. Because we are applying a square wave to the primary, this is easy to calculate. The area under the curve of the square wave applied to the transformer is given by Equation 1. To make sure the transformer never saturates, the VS must be calculated to account for the worst-case condition.

Equation 1. V S =   V D D × t o n

Droop is the gate voltage loss due to magnetizing current. This voltage drop is given by Ohm’s law, so we need to find the magnetizing current and the resistance. Magnetizing current is the current from the transformer primary inductance. When a voltage is applied across the primary of the transformer, a current builds according to the following equation:.

Equation 2. V = L × d i d t

The magnetizing current is one component of the total current in the primary. The other current components are the gate drive current, and the current into the base of the PNP.

GUID-20240226-SS0I-MK1H-PKVR-HM6KT6PGBQXF-low.svg Figure 4-1 Simulation Showing the Major Current Components in the Gate Drive Circuit

In Figure 4-1, the total current appears very similar to the gate drive current, because that is the largest component. However, there is still a magnetizing current circulating through the primary. Using Equation 2, we can calculate this current. By re-arranging the terms we find:

Equation 3. ∆ I =   V D D L m a g × t o n

The current across the primary rises and falls by ΔI as the polarity of VPRIME alternates. This current is centered around 0, so the peak current is given by:

Equation 4. I p e a k = ∆ I 2

As dead-time approaches 0, the magnetizing current becomes a triangle wave centered around 0V. The root-mean-square (rms) of a triangle wave is given by:

Equation 5. I r m s =   I p e a k 3

By substituting all of these terms, we can build an equation for the rms of the magnetizing current:

Equation 6. I r m s = V D D × t o n L m a g × 2 × 3

The droop is given by the following equation:

Equation 7. V d r o o p = I p e a k × R p r i m a r y

By substituting the Ipeak equation from before we find that:

Equation 8. V d r o o p = V D D 2 × L m a g × t o n × R p r i m a r y

Designers often target a droop of less than 5%. This can also be written in terms of VDROOP and VDD:

Equation 9. V d r o o p V D D ≤   1 20

By substituting this into the equation for VDROOP, the simplified equation becomes:

Equation 10. L m a g ≥   10 × t o n × R p r i m a r y

By meeting this condition, the droop is kept under 5%. In this case, RPRIMARY is the sum of all resistances in the primary loop. The main components of RPRIMARY are as follows:

Equation 11. Rprimary=ROH+ROL+Rloop+Rprimary

When the driver is in the push-pull configuration, the current flows through the pullup of one output and the pull-down of the other output. See Figure 2-1 for an illustration that shows this effect. In addition to the resistance of the output stage, any additional resistance from discrete resistors and the transformer primary coil resistance adds to the total resistance.