HIP2103, HIP2104

6. Application Examples

FN8276 Rev.2.0

Page 23 of 30

Feb 25, 2021

The value of RHS is determined by how much average current in the clamping diode is acceptable. Current in the

low-side FET flows through the body diode during the dead time resulting with a negative voltage on HS that is

typically about -1.5V. When the low-side FET is turned on, the current through the body diode is shunted away

into the channel and the conduction voltage from source to drain is typically much less than the conduction

voltage through the body diode. Consequently, significant current flows in the clamping diode only during the dead

time. Because the dead time is much less than the low-side FET’s on-time, the resulting average current in the

clamping diode is very low. The value of RHS is then chosen to limit the peak current in the clamping diode and

usually just a few ohms is necessary.

The methods to clamp the negative transients with diodes can still result with high frequency oscillations on the

HS node depending on the parasitics of the PCB design. An alternative to the clamping diode in Figure 26 is a

small value capacitor instead of the diode. This capacitor and RHS is very effective for minimizing the negative

spike amplitude and oscillations.

However, this solution also has its limitations. Depending on the value of the filter capacitor and the PWM

switching frequency, RHS can dissipate significant power because the voltage on the capacitor is switching

between the bridge voltage and ground. Usually, the power dissipated by RHS is small because the switching

frequency for most motor drives is <20kHz and the value used for Cfilter is typically about 1000pF.

Another issue is that the charge on Cfilter is partially transferred to the gate of the high-side FET when the low-side

FET turns on. When the phase node goes low, a voltage is impressed across RHS as shown in Figure 27.

Because HO is low, the voltage across RHS is also across the gate of the high-side FET. If the filter capacitor is

very large, the voltage on the gate approaches the bridge voltage turning on the high-side FET resulting with

shoot-through. Fortunately, the voltage across RHS is much less than the bridge voltage for two reasons. First, the

voltage across RHS is determined by the turn-on time of the low-side FET. As the low-side FET is turning on, the

charge on the filter capacitor is depleting lessening the voltage across RHS. Also, because the relatively large gate

capacitance of the high-side FET is in parallel with RHS, the voltage impressed on the gate is further reduced. In a

Figure 26. Resistor and Diode Negative Transient Clamp

Figure 27. Resistor and Capacitor Negative Transient Filter

VSS

HS

LO

HO

Inductive

Load

+

-

+

-

RHS

HB

CBOOT

VSS

HS

LO

HO

Inductive

Load

RHS

HB

CBOOT

+

_

Cfilter

IBAT, 0kHz