AN1524 Freescale Semiconductor / Motorola, AN1524 Datasheet - Page 5

• Manufacturer Part Number: AN1524
• Description: AC Motor Drive Using Integrated Power Stage
• Manufacturer: Freescale Semiconductor / Motorola

voltage diode (D4), a small resistor (R46), and capacitor
(C18). When the lower IGBT is turned on, the capacitor
charges through the diode and resistor. The lower transistor
is then turned off and the upper transistor is turned on. The
charge stored in the capacitor supplies the gate drive energy.
Actually, the bias current of the IC and optocoupler pull–up
resistor are the primary current drains on the bootstrap
capacitor. The 10 F capacitor can provide a holdup time of
several milliseconds. An 18 volt supply is recommended
because about 3 volts are lost in the lower transistor on
voltage and the bootstrap diode. The small resistor limits the
peak current under transient conditions.
high current MOSFET driver. This is an economical LVIC with
1.5 amps of source and sink capability. Separate resistors are
used for turn–on and turn–off. A low turn–off impedance is
essential to minimize turn–off losses and shoot–through
current. The turn–on resistor is selected to limit the turn–on
dv/dt to an acceptable level. The values for R52 and R53 thus
vary according to the specified power module/development
system.
losses in the IGBTs, particularly for the high voltage drives.
The dv/dt applied to the motor is also a consideration as this
stresses the motor insulation. The typical dv/dt measured on
the demo board is 5 to 10 V/ns during IGBT turn–on. This
value can be lowered by adding output filter inductors or using
(U1 PIN 4)
(U1 PIN 5)
MOTOROLA
Power is supplied by a bootstrap circuit consisting of a high
The gate drive current is supplied by an MC33151D (U11)
The dv/dt limitations necessitate some additional power
MinusI
PlusI
1.10 k
1.10 k
R26
R27
+ 2.5 V
R25
13.0 k
C9
120 pF
Figure 6. Current Amplifier and Over–Current Comparator
Freescale Semiconductor, Inc.
For More Information On This Product,
C10
120 pF
Go to: www.freescale.com
MC33272D
+
–
13.0 k
120 pF
R30
C11
U4b
larger gate drive resistors. However, using larger gate drive
resistors will greatly increase power dissipation in the IGBTs.
optocoupler. Two non–inverting MC33152Ds are used for the
three low side transistors and the brake transistor.
Status Circuitry
between the brake and inverter. This allows the resistor to
sense both positive motor current and dynamic braking
currents. A resistor placed between the rectifier and brake
transistor would not sense the regenerative current when the
brake is turned on. The DC link current may be used in many
different control and protection schemes. The DC current in
this resistor multiplied by the DC voltage gives the real power
output of the inverter. The real power divided by 3 and divided
by the output phase voltage gives the real output phase
current. Unfortunately, the output power factor is not known.
However, the peak current in the resistor is indicative of the
peak phase current. Thus, a DC measurement and a peak
over current detect provides a cost effective means of control
and protection.
shown in Figure 6. A differential amplifier senses current in
both directions. A 2.5 volt reference provides an offset for the
differential amplifier. The conditioned output varies from 0 to
5 volts with 2.5 volts representing zero current. The
over–current comparator trips at 12.7 amps.
The low side gate drive uses a similar circuit without the
The sense resistor in the integrated power stage is placed
The current amplifier and over–current comparator are
+ 5 V
R29
12.1 k
R28
3.01 k
(4.0 V)
+
–
LM293AD
U3b
+ 5 V
R31
2.2 k
IO
(ANALOG OUTPUT)
OC
(TO PLD)
AN1524
5