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EiceDRIVER™ gate driver IC: Non- isolated solution to drive high-side power supply in half bridge configuration- KBA236398

EiceDRIVER™ gate driver IC: Non- isolated solution to drive high-side power supply in half bridge configuration- KBA236398

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In continuation to KBA236394, the following section is discussed briefly.

This circuit consists of a bootstrap capacitor, a resistor, and a diode in series. This circuit is used to supply the bias voltage for high-side switch in order to operate the top switch normally, when the source/emitter point transits. It is one of the simple and low-cost solutions to drive the top switch.

The main goal is to avoid very high gate requirements with respect to the emitter or source terminal, when the high switch is operated. To meet the requirement, we keep a capacitor on the high side, which supports the gate voltage by giving a proper reference even during transitions. When the bottom-/low-side switch is ON, the bootstrap capacitor charges as per the set voltage level. During the low-side switch OFF, the charged capacitor discharges some of its energy to the high-side switch and hence helps to gain a higher required voltage at the gate referred to as source or emitter.

The below figures show the charging and discharging path of the capacitor when the low-side switch is ON/OFF.

Figure 1 The capacitor's charging path.pngFigure 1 The capacitor's charging path

Figure 2 The capacitor's discharge path.png

 Figure 2 The capacitor's discharge path

The main reason for choosing two different ICs (Half bridge gate driver IC products) in the above figures is to familiar the different nomenclature used for the same pins. The pin naming analogy is shown below for reference.

Vdd = Vcc pin, Vb = Hb pin, and Vs = Hs pin in the figure shown above.

Key points while choosing a capacitor

  • If the capacitor is not properly designed, it may result in duty cycle limitation and power switch on time. The dynamic response relation between time constant of bootstrap circuit (C, R) and system response time shows the following effects:
    - At low duty ratio operation, the bootstrap time constant is higher and thus makes the system response slower.
    - Similarly, for higher duty ratio, the response is faster.
     Below equation shows the relation:

equation.png…………………. eq-1

  • The charge required by the bootstrap capacitor comes from a large bypass capacitor which is connected across the Vdd/Vcc pin. As a general rule, the bypass capacitor should be ten times more than the bootstrap capacitor so that the Cap.VDD/Cap.VCC doesn’t discharge completely during the bootstrap charging time.
  • It is recommended to use low ESL and ESR value, low thermal coefficient and tolerance SMD MLCC capacitors with a voltage rating at least double of Vcc/Vdd.
  • For under-designed capacitor values, there is a high chance of UVLO triggering and for over-designed value, ripple voltage will be lower with a longer reverse recovery period during the initial or short charging time along with a high charging current.
    The relation between charging current and the bootstrap capacitor is given below:

equation.png…………………………eq-2

Key notes while choosing the bootstrap resistor

  • The role of the resistor is to limit the charging current of the bootstrap capacitor while starting and limits the dV/dT across the Vb-Vs pins. It should be carefully selected, otherwise it will affect the dynamic response, which was stated earlier in the capacitor section. From the equation -1, it is clearly observed that while starting Cboot = 0 V, if the resistor value is high, the charging time will be longer, and hence the start-up time will be more.
  • The resistor chosen should be able to withstand the energy dissipated during the start-up conditions and should have a low inductive value. The energy can be estimated by using the below formula.

equation.png………… eq-3

key notes while choosing the bootstrap diode

  • The purpose of the diode is to stop any reverse direction conduction, i.e., from Vboot to Vcc/Vdd. It is important to choose a diode that has a low reverse recovery time, low capacitance across the PN junction, and a low voltage drop.
  • A general recommendation for choosing such a diode is the Schottky diode (CooL SIC schottky diode) or any other fast recovery diode.
  • The voltage rating for such a diode should be higher than the DC link voltage because during high-side switch ON the emitter or source point goes to +DC voltage and the diode experiences a high reverse voltage and should be fast enough to reverse bias itself.
  • During transitions, especially when starting, the diode experiences high transient current and should be capable of handling such conditions.
  • If the diode exhibits oscillations during reverse recovery, it can amplify other paracitics in the circuit layout and cause a high oscillation across the Vs and Vb pins, resulting in UVLO protection or, in some cases, can damage the ICs.
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