TN90003

The Qrr of a MOSFET: its importance in motor-control circuitdesigns
Rev. 1.0 — 21 July 2020

technical note

Document informationInformation

ContentKeywords

MOSFET, reverse recovery charge, Qrr

Abstract

When choosing a MOSFET for a motor-control application, design engineers tend to first study
the device’s on-resistance characteristic, since this parameter has the greatest effect on theMOSFET’s ability to provide sufficient current to the motor without generating excessive heatcaused by conduction losses. Some other aspects of the MOSFET’s operation also have amarked effect on thermal performance, and attention paid to these factors can offer worthwhile
rewards in improved motor-drive design features..

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The Qrr of a MOSFET: its importance in motor-control circuit designs

1. Introduction
Motor-drive circuits generally switch at lower frequency (typically 20 kHz - 40 kHz) than switch
mode power supplies (SMPS, normally 100+ kHz). This often induces designers to ignore thethermal contribution from MOSFET dynamic switching losses attributable to parameters suchas input and output capacitance, since they are assumed to be much smaller than the effect ofconduction losses attributable to on-resistance.
In motor-control applications, however, in which the motor and controller are often physicallyconnected by wires, design engineers are increasingly taking account of EMC (Electro-MagneticCompliance) at the component selection phase. The non-ideal switching behavior of a MOSFET
can have a marked effect on electro-magnetic emissions: problems caused by these emissions
might only become apparent at the qualification phase of the design project.
Furthermore, the efficiency of the motor drive does not have an important effect on systemefficiency since motors are inherently less efficient than motor drives: 80-85% efficiency is typicalfor a motor, whereas a drive circuit’s efficiency often exceeds 95%.
MOSFET losses in the drive circuit do however greatly affect the operating temperature of thecontrol circuit’s PCB (Printed-Circuit Board), and since the MOSFET’s on-resistance increases astemperature rises, then excessive heating must be avoided through good design and component
selection. As OEMs (Original Equipment Manufacturer) strive to meet consumer demand forsmaller, more powerful products such as hand-held power tools and domestic appliances,MOSFETs which generate less waste heat can enable the designer to achieve reductions in size,
weight and cost as well as to extend battery run-times.

2. Adding a gate resistor
To reduce electro-magnetic emissions in motor-control applications, it is common practice to add aresistor in series with the MOSFET’s gate. When combined with the MOSFET’s capacitive gate, theresistor makes a low-pass filter which helps to reduce the switching speed of the MOSFET. Thiscan be an effective means of reducing high-frequency harmonics, which contribute to the circuit’selectro-magnetic emissions. But designers must be careful not to add too much resistance at theMOSFET’s gate, since reducing the switching speed extends the time during which the MOSFET
operates in linear mode, causing it to dissipate more waste energy.
The heating effect of adding a 47 Ω gate resistor can easily be observed with a thermal camera, asshown in Fig. 1 and 2.

gate resistor = 0 Ω; load current = 3 A
Fig. 1.

MOSFET temperature = 61.5 °C

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technical note

gate resistor = 47 Ω; load current = 3 A
Fig. 2.

MOSFET temperature = 65.6 °C

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The Qrr of a MOSFET: its importance in motor-control circuit designs

3. The importance of reverse-recovery charge
A lesser understood parameter, reverse-recovery charge, Qrr, is also known to make a largecontribution to electro-magnetic emissions. During the dead-time in a motor-drive circuit, when both
MOSFETs are turned off, free-wheel current flows briefly through the body diode of the low-side
MOSFET: stored charge, Qrr, accumulates across the body diode’s PN junction, as shown in Fig. 3.

Fig. 3.

Reverse-recovery charge accumulates across the body diode’s PN junction

When the high-side MOSFET turns on, a current surge occurs, and flows briefly through the highside and low-side MOSFETs, until the accumulated charge of the low-side is removed, as shownin Fig. 4. IRM (peak recovery current) can be relatively of high values and contributes to switchinglosses. This high-speed pulse also contributes markedly to electro-magnetic emissions.

Fig. 4.

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Surge current in the high-side MOSFET caused by Qrr in the low-side MOSFET

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The Qrr of a MOSFET: its importance in motor-control circuit designs

Low Qrr – reduced body diode reverse recovery current Irr
Fig. 5 shows the SPICE simulation results for 2 same devices, one of which was modelled to have
twice the amount of Qrr.

Fig. 5.

SPICE simulation result

The SPICE simulations show the benefits of low Qrr in switching applications, Irr reduced by 50%
for low Qrr types. In Fig. 5 the shaded area represents the additional energy loss for parts with 2 xQrr.
Nexperia NextPower 100 V types have a higher softness factor (tb / ta > 1) - gives lower ringing /resonance and lower spike voltage. Lower Irr, ringing resonance also contributes to lower EMI
emissionsEMC
The Irr pulse interacts with the MOSFET’s lead-frame, and with the PCB’s parasitic inductance andcapacitance, to create a resonant circuit. The Irr effect when combined with parasitic or package
inductance will typically resonate at around 60 to 130 MHz.
Resonance can be reduced by carefully reducing the PCB’s parasitic inductance through good
PCB design, adding a snubber circuit, or by choosing MOSFETs with a low Qrr rating.
The spectrum analyzer trace, shown in Fig. 6, compares the measured RF emissions for a low-Qrr
NextPower 100 V MOSFET from Nexperia with those of a competitor’s MOSFET the Qrr of whichis twice as high. In this example, the NextPower 100 V low-Qrr technology offers a reduction inelectro-magnetic emissions of around 6 dB at frequencies between 70 MHz and 110 MHz. Use of
the Nexperia low Qrr part thus markedly reduces the risk of encountering EMC problems during
qualification of a motor-drive design.

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The Qrr of a MOSFET: its importance in motor-control circuit designs

Fig. 6.

Reduced electro-magnetic emissions from a Nexperia NextPower MOSFET which
has low Qrr

4. Revision history
Table 1. Revision history
Revision
Date
numberDescription

1.0

intitial version.

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2020-07-21

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The Qrr of a MOSFET: its importance in motor-control circuit designs

5. Legal information
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Notwithstanding any damages that customer might incur for any reason
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The Qrr of a MOSFET: its importance in motor-control circuit designs

List of Tables
Table 1. Revision history......................................................5TN90003

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The Qrr of a MOSFET: its importance in motor-control circuit designs

List of Figures
Fig. 1. MOSFET temperature = 61.5 °C.............................. 2Fig. 2. MOSFET temperature = 65.6 °C.............................. 2
Fig. 3. Reverse-recovery charge accumulates acrossthe body diode’s PN junction............................................... 3
Fig. 4. Surge current in the high-side MOSFET causedby Qrr in the low-side MOSFET.......................................... 3
Fig. 5. SPICE simulation result............................................ 4
Fig. 6. Reduced electro-magnetic emissions from a
Nexperia NextPower MOSFET which has low Qrr...............5

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The Qrr of a MOSFET: its importance in motor-control circuit designs

Contents
1. Introduction................................................................... 2
2. Adding a gate resistor................................................. 2
3. The importance of reverse-recovery charge.............. 3
4. Revision history............................................................5
5. Legal information......................................................... 6© Nexperia B.V. 2020. All rights reserved

For more information, please visit: http://www.nexperia.com
For sales office addresses, please send an email to: salesaddresses@nexperia.com
Date of release: 21 July 2020

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