MIC4609 datasheet(23/34 Pages) MICROCHIP

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 2016 Microchip Technology Inc.

DS20005531A-page 23

MIC4609

5.0

APPLICATION INFORMATION

5.1

Bootstrap Circuit

The high-side gate drive cannot be operated

continuously (100% duty cycle). It must be periodically

turned off to refresh/recharge the bootstrap capacitor,

CB. There are two separate requirements to consider

when choosing the bootstrap capacitor value:

• IGBT or MOSFET gate charge

• Duration of the high-side switch on-time

The high-side bootstrap circuit for Phase A is illustrated

in Figure 5-1.

FIGURE 5-1:

MIC4609 – Bootstrap Circuit.

The bootstrap capacitor voltage drops each time it

delivers charge to turn on the IGBT. The voltage drop

depends on the gate charge required by the IGBT. Most

IGBT and MOSFET specifications contain gate charge

versus VGE or VGS voltage information or graphs.

Based on this information and a recommended

VHB of

0.1V to 0.5V, the minimum value of bootstrap

capacitance is calculated by applying Equation 5-1.

EQUATION 5-1:

After the high-side switch has turned on, the bootstrap

capacitor will continue to discharge due to leakage

currents in the bootstrap capacitor, the IGBT/MOSFET

gate-to-source and the driver (HS-pin-to-ground

leakage).

Typical leakage currents for the bootstrap capacitor

and IGBT/MOSFET are in the 100 nA range. The

MIC4609 HS-pin-to-driver leakage current is generally

higher with typical values in the 1 µA range (or higher

at high junction temperature and voltage). The

minimum value of bootstrap capacitor that prevents an

excessive drop in the gate drive voltage to the

high-side switch is calculated as per Equation 5-2.

EQUATION 5-2:

Resistors RHB and RCB can be used to reduce the peak

CB charge current or modify the high-side

IGBT/MOSFET turn-on time. This helps reduce noise

and EMI as well as ripple on the VDD pin.

The resistor in series with the HB pin, RHB, controls the

turn-on time of the high-side switch by limiting the

charge current into the gate.

Adding a resistor in series with capacitor CB will reduce

the peak charging current drawn through diode DBST. It

has some effect on slowing down the high-side switch

turn-on time, however, it is not as effective as resistor

RHB since charging current also comes from VDD until

the high-side switch starts to turn on and raise the

voltage on the HB node.

5.2

HS Node Clamp

A resistor/diode clamp between the switching node and

the HS pin is recommended to minimize large negative

glitches or pulses on the HS pin.

As shown in Figure 5-2, the high-side and low-side

IGBTs turn on and off to regulate motor speed. During

the on-time, when the high-side IGBT is conducting,

current flows into the motor. After the high-side IGBT

turns off, and before the low-side IGBT turns on, there

is a brief period of time (dead time) that prevents both

IGBTs from being ON at the same time. During the

dead time, current from the motor flows through the

diode in parallel with the low-side IGBT. Depending on

the diode characteristics (VF and turn-on time), the

motor current and circuit parasitics, the initial negative

voltage on the switch node can be several volts or

more.

Even though the HS pin is rated for negative voltage, it

is good practice to clamp the HS pin with a resistor and

diode to prevent excessive negative voltage from

damaging the driver. Depending on the application and

AHS

AHB

AHO

VDD

ALO

AHI

ALI

COM

VIN

RHS

DCLAMP

DBST

CVDD

*RCB

*RHB

* Optional Components

Level

shift

QGATE

V

-----------------



Where:

QGATE = Total gate charge at VHB

VHB = Voltage drop at the HB pin

Where:

tON = Maximum ON-time of the high-side

switch

VHB = Voltage drop at the HB pin

Idischarge = Total discharge current at the HB pin

(capacitor, IGBT/MOSFET, and HB

pin)

tON Idischarge



V

-----------------------------------



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