SLVA504A February   2012  – July 2021

1. 1Sources of Power Dissipation in an Integrated Driver
1. 1.1 H-Bridge
2. 1.2 Half Bridge
2. 2Example Calculation
1. 2.1 H-Bridge
2. 2.2 Half Bridge
3. 3References
4. 4Revision History

# 1 Sources of Power Dissipation in an Integrated Driver

There are two fundamental sources of power dissipation on the power FETs in a driver IC.

1. Power dissipation from conduction loss of each FET due to its on-resistance is given by:
Equation 1. PRON [W] = RON × IL2, where,

1. RON = FET on-resistance [ohm]
2. IL = Load current [A]
Note that RON increases with temperature. So as the device heats up, the power dissipation also increases. This must be considered when calculating the total device power dissipation. Typically, the RON approximately doubles its value at 150 Cº compared to room temperature at 25 Cº.

2. Power dissipation due to switching losses associated with PWM based current regulation can be approximated with the following expressions:
1. Power dissipation due to output slewing during rising and falling edges is given by:
Equation 2. PSW1 [W] = (0.5 x VM x IL x VM / SRrise x fPWM) + (0.5 x VM x IL x VM / SRfall x fPWM), where,

1. fPWM = PWM switching frequency [Hz]
2. VM = Supply voltage to the driver [V]
3. IL = Load current [A]
4. SRrise = Output voltage slew rate during rise [V/sec]
5. SRfall = Output voltage slew rate during fall [V/sec]
Output slewing rate is a balance between EM (Electro magnetic) performance and device power dissipation.

2. Power dissipation due to the dead times between switching FETs is given by:
Equation 3. PSW2 [W] = (VD x IL x tDEADrisex fPWM) + (VD x IL x tDEADfallx fPWM), where,

1. fPWM = PWM switching frequency [Hz]
2. VD = FET body diode forward bias voltage [V]
3. IL = Load current [A]
Dead times are necessary to mitigate any risk of current shoot through between the switching power FETs. Integrated FET drivers often have a feedback based self timed FET switching sequence to ensure the smallest possible dead times while avoiding any shoot through current.

3. Power dissipation due to OUTPUT slewing during FET turn ON in the recirculation path is given by:
Equation 4. PSW3 [W] = (0.5 x VD x IL x VD / SRrise x fPWM) + (0.5 x VD x IL x VD / SRfall x fPWM), where,

1. fPWM = PWM switching frequency [Hz]
2. VD = FET body diode forward bias voltage [V]
3. IL = Load current [A]
4. SRrise = Output voltage slew rate during rise [V/sec]
5. SRfall = Output voltage slew rate during fall [V/sec]
This dissipation is typically not considered as it is quite insignificant.

4. Power dissipation also occurs due to reverse recovery losses of switching FET. This occurs due to change in current direction of the forward biased body diode of a typically large power FETs (RDSON < ~100 mΩ). These losses typically limit the power dissipation savings at the higher slew rates (> 25 V/μsec). This dissipation is also typically not considered as it is quite insignificant.
3. Power dissipation due to device current consumption, given by,
Equation 5. PIVM [W] = VM x IVM, where,

1. VM = Supply voltage to the driver [V]
2. IVM = Device operating supply current [A]
This dissipation is typically not considered as it is quite insignificant, given that IVM is typically ~5 - 10 mA.

4. Some driver devices have an external LDO regulator output available that is used to provide some reference current, or current to power external loads. Power dissipation due to this external load current is given by,
Equation 6. PLDO [W] = (VM – VLDO)x ILDO, where,

1. VM = Supply voltage to the driver [V]
2. VLDO = LDO output voltage [V]
3. ILDO = External load current [A]

This dissipation is typically not considered as it is quite insignificant.

In summary, power dissipation total is given by:

Equation 7. PTOT = PRON + PSW1 + PSW2 + PSW3 + PIVM + PLDO

Typically, this can be approximated to just three sources, given by:

Equation 8. PTOT = ~(PRON + PSW1 + PSW2)

The next set of sub-sections show the power dissipation in each power FET for conduction and switching losses based on the application configuration (H-bridge or Half bridge driver using high-side or low-side recirculation).