Soft Start Overvoltage and Current Limit Circuits

TM 11-6130-429-40

trols the speed of response of the current limit circuit

(2) When the inverter is running, an auxiliary

secondary on transformers T1 and T2 generates a

so that it is stable, and R22 discharges the soft start

-6V power source by the action of CR15, CR16, and

capacitor C5 so that if a power supply is short cir-

cuited, the soft start is utilized in the restart.

C7 which is used for turning off the power transistors

as follows: When the pulse from U1 is terminated,

(2) When the output voltage is present, U4 is

turned on, setting the current limit at the normal

Q16 is turned on, thereby pulling the base of Q8

value. If the power supply is shorted, U4 is turned

negative while simultaneously turning off Q7. This

off, which reduces the current limit reference and

provides a reverse bias drive to the base of Q1, assur-

causes foldback under short circuit conditions. Ca-

ing fast turnoff. Zener diode VR3, in conjunction

with R10, limits the negative voltage to which the

pacitors C6 and C13 prevent noise or spurious activa-

base of Q1 can be driven, and resistor R18 deter-

tion of the current limit circuit.

mines the reverse current capability of the drive. On

(3) U2B is the overvoltage detector chip which is

normally biased in the off position by resistive di-

the alternate half cycle, Q6 activates Q9, Q10, and

Q17 and drives Q4 on and off in the same manner.

viders R31-R27 and R41-R42. In the event of any

i. Soft Start Overvoltage and Current Limit Cir-

overvoltage condition in the output, light-coupled

transistor U5 is activated, thereby turning on the

cuits (fig. 1-9).

overvoltage circuit, which simultaneously turns off

(1) Integrated circuit U2 contains four voltage

comparators. The output of each comparator is a

pin 9 and discharges soft start capacitor C5. CR10

transistor which is turned on or off. U2C is the cur-

latches the overvoltage to the on position until the

rent limit detector which is activated by the second-

input voltage is temporarily interrupted, which gen-

erates a reset.

ary of current transformer T1. The output of the full

(4) U2A is the soft start circuit which monitors

wave bridge rectifier CR11 through CR14 is propor-

the 17V bus by way of resistive divider R39-R40 and

tional to the current flowing in the two inverter

compares it with the voltage reference R30-R28.

transformers and appears across R38. This voltage is

When the bias reaches approximately 15V, U2 is

compared with a fixed current limit reference derived

turned off, thereby allowing the soft start to begin. If

f r o m the 5V reference by resistive divider

R37-R35-R36. In the event that the instantaneous

the bias drops for any reason, U2A turns on, stop-

current in either half wave forward converter exceeds

ping the pulser through CR5 and resetting the soft

start through CR6 and R21. Resistor R26 creates a

the prescribed amount, U2 turns on, thereby pulling

hysteresis in the bias detection level which elimi-

down pin 9 of U1 through CR8, which narrows the

nates chattering during turn on and turn off.

pulse width in a current limiting fashion. C18 con-

1-17