Tektronix TM 5006 Theory of Operatio n-ΤΜ, Wave bri dge rectif ie r fo r 220 V ac li ne, Full

Theory of Operatio n-ΤΜ 5006

Full wave bri dge rectifier fo r 220 V ac line. Voltage doubler for 110 V ac line

2950-13

Fig. 32. Input line co nnections fo r 110 V a nd 220 V operation.

When pi ns 1 of U1950A and 13 of U19508 are low, no drive is applied to the output stage. Wit h one gate out put hig h and t he ot her low, base drive is applied to one out put transistor. I nput pins 4and 11 co nnecttot hewait flip-flo p, U1940Α. Ι nput -pi ns 3 and 12 co nnect to 019408, the dead ti me multivibrator. I nput pi ns 5 and 10 connect to the output of the stop monosta ble, U1951A. Pi ns 2 and 9 co nnect to the co mpleme nt outputs of the divide- by-2flip- flop, U1951 B. W it h any one or α11 of these inputs hig h, the output li nes are Ιοω αηd no drive is applied to one or bot h of the output transistors.

The dea d time multivi brator ci rcuit ry determi nes the minimum off time of the output transistors. Dead time is necessary to allow one output transistor to co mpletelyi

turn off before t he ot her turns on. At start up, the Α nput (pin 12) of 019408 goes low. This allows U1940Btotrigge r on the cloc k signal at the Β in put (pi n 11). The mi nimum ti ming period of U1 940B, determi ned by R 1930and C2040, is 5 μs. This pulse wi dth is le ngt hened by C2050, CR2040 and CR2043 as the voltage on C2051 and C2060 is decreased. The pulse wi dth of the power supply output varies fo r soft start and power limit. When pin 10 of U1940B is hig h, bot h output transistors are off.

Dead time or output t ransistor offtimeis maxim um with C2051 and C2060 discharge d and minim um charge d. The out put power availa ble gradually increases as t hese ca pacitors charge duri ng soft start. The soft start keeps hig h i nput currents to ca pacitors from damagi ng ci rcuit components . When pin 6 of the wait multivi brator U1 940A goes hig h for any reason (fault), pi n 8 of U 1840D goes low dischargi ng C2051 and C2060. Und er normal operatio n, when power is turned on, C2051 and C2060 charge to the voltage at pi n 8 of U1840D as determi ned by R 1953 and R2060 . This takes appr oximately 1/2 seco nd.

The power li mit co ntrol is R 2060 . For maximum power, this co ntrol must be in the fully clockwise positio n ( maxim um resista nce). For servici ng the control ca n be adjusted for reduced power output levels . This is ac- co mplished by reducing the resistance of R 2060, li miti ng the voltage ac ross C2051 an d C2060.

The pur pose of flip-flo p U1951 A isto varyt he οη ti me of t he out put transistors co nsistent with the output voltage level. When pi n 6 of U1951A goes hig h , pin 1 also goes hig h. This action shuts dow n the base drive ci rcuit ry, red uci ng power output. The lengt h of ti me pi n 6 remai ns hig h is co ntrolled by the O utp ut R eg ulator ci rcuit ry. The risi ng portio n of the wavefo rm at pi n 4 of U1951A resets the flip-flo p fo r α low co nditio n at pin 1 .

Flip-flop U 1951 B divides the 40 kHz out put wavefo rm from U1940 B to 20 kHz. The pulse from t he dea d ti me multivibrator, U 1940 B, is applied totheclock termi nal (pi n 11) of 019518 . The O terminal of U1951 Β is co nnecte d to its Dinput. The multivibrator U1951 Btoggles on the rising edges of the dea d time multivibrator (0194013) output.

Output Regulator

The 40 kHz cloc k oscillator, whic h provi des the basic timi ng necessa ry fo r the co ntrol ci rcuit ry, is co mposed of U1840A, Β and C. Feedbac k goes from pi n 6 of U 1840C and pi n 4 of U18408t ο ρίη 1 of U1840 Α. The output voltage at pi n 4 of U 1840 B is hig h fo r about 4 μs and low about 21 μs. This non-symmetrical d uty cycle is accom plished by CR1850 and is necessa ry fo r proper operatio n of U 1940 B.