20 kHz Switching Converter

TM 11-6130-429-40

the drive waveforms that the two converters are op-

(3) Rectifier CR1, in effect, shorts the input bus

if reverse voltage is applied and accordingly trips the

erating 180 out of phase.

system circuit breaker and prevents damage to the

(5) The components shown on figure 1-3 are

power supply. Blower B1 provides cooling air for the

mounted on the main frame assembly with the excep-

power supply and the system.

tion that the components shown within the dashed

(4) The front panel ON-OFF switch applies

lines are located on printed circuit board (PCB) A2.

power to the control circuits which in turn activate

The power transistors which activate transformer T1

the 20 kHz switching converter. When switch S1 is in

are Q2 and A3 operating in parallel. The network

the off position or if the power supply is not operat-

L1A-CR1-R1-L1B-CR2-R2 on PCB A2 assures

ing properly, the front panel fault light DS1 is acti-

current-sharing between Q2 and Q3 during the on

vated.

pulse. In effect, L1A and L1B operate as an inter-

b. 20 kHz Switching Converter (fig. 1-3).

phase transformer so that if Q2 is conducting more

(1) Power transistor Q2 is turned on and off by

heavily than Q3, the emitter of Q3 will be pulled

external drive circuits with the waveform as shown

negative by transformer action to assure that the

in figure 1-3. When the input voltage to the base of

currents are equalized. CR1-R1-CR2-R2 assures re-

Q2 is positive, the transistor conducts and current

set during the off time. Transformer T1 on PCB A2 is

flows through the primary of T1P1. The voltage on

a current transformer which detects the instantane-

the collector of Q2 decreases to the saturation volt-

ous current flowing within either set of switching

age of the transistor (on the order of 1V).

transistors. The secondary of this current trans-

(2) By transformer action, as current flows in

former is monitored, and if for any reason the current

the primary, it also flows in the secondary of T1

in the switching transistors exceeds the predeter-

through filter capacitor C15 and filter choke L3.

mined safe value, the pulse is narrowed to limit the

When the pulse on the base of Q2 ends, the voltage on

current at all times independent of line, load, or fault

T1P1 goes up to approximately twice the input line

conditions to a safe value.

voltage. When this occurs, transformer primary

(6) The two power switching transistors are

driven by a Darlington transistor with a fast turnoff

T1P1 conducts through CR5, which delivers the

magnetizing current in the primary of T1 back to the

diode (Q1-CR2,Q4-CR4). The Darlington transistor

line in a dissipationless fashion. This limits the volt-

reduces the amount of drive current required by the

age on Q2 to approximately twice the input line volt-

switching transistors, and the fast turnoff diode per-

age. The voltage on the collector of Q2 remains in this

mits back bias of the switching transistors during

condition until the magnetizing current is reduced to

the turnoff mode to reduce the storage time.

approximately zero, at which time the voltage on the

(7) The reset winding of power transformer T1 is

collector of Q2 returns the line voltage as indicated.

tapped at terminal 4, and the winding between termi-

The transformer has now been "reset" and awaits the

nals 4 and 5 is rectified and filtered by CR15, CR16,

next pulse.

and C7 to generate approximately 6V. This voltage is

(3) The waveform on the secondary of T1S1 is

subsequently used in the control circuits for reverse

identical in shape to the voltage on the primary, but

biasing the switching transistors during the turnoff

reversed in polarity and adjusted in magnitude by

period.

the turns ratio. Rectifier CR6 permits current to flow

(8) During the interval when the switching tran-

to the load when Q2 is conducting, which in turn

sistors are either turning on or off, there will be com-

causes current to build up in filter choke L3. When

mutation losses since the voltage across the transis-

Q2 stops conducting, CR6 blocks the negative pulse

tor may be high while currents are simultaneously

and the current flowing in L3 conducts through

flowing in the transistor. These losses are minimized

CR10 until the next pulse occurs. Accordingly, the

by the commutation network consisting of C3, CR18,

action of the choke is to convert the pulsating output

CR19, CR17, and CR16. Operation of the commuta-

voltage into a continuous DC current. Output filter

tion network for transformer T1 is as follows. When

Q2 and Q3 are turned off at the end of the pulse, the

capacitor C15 smooths the output voltage and elimi-

nates ripple and noise from the output. If the pulse

magnetizing and residual currents flowing in the pri-

width is increased, the average voltage at the output

mary of T1 flow into capacitor C3 and then returned

will increase, and accordingly, by controlling the

to the positive bus. During this phase, C3 is charged

pulse width, the output voltage may be regulated

to approximately the input line voltage. When Q2

independent of line and load conditions.

and Q3 are turned on at the beginning of the next

(4) The voltage on EMI filter capacitors C4 and

pulse, the cathode of CR18 is be driven negative by

C5 supplies power to two half-wave forward con-

an amount approximately equal to the line voltage.

verters which, in turn, activate the primaries of T1P1

This action causes current to flow through CR18 and

and T2P2, respectively. It will be noted by review of

inductor L5 in a "resonant" charging action, which,

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