Application of thermally compensated optocouplers in voltage converters. Part 2. Scheme, description

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Application of thermally compensated optocouplers in voltage converters. Part 2. Encyclopedia of radio electronics and electrical engineering

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The simplest analog optocoupler decoupling applicable in automobile PNs is the divider-optocoupler-ION. A fragment of a real PN for powering a single-cycle tube (in class A) is given. The support on the zener diode (39V) practically eliminates the influence of the temperature instability of the optocoupler. But at what cost: at the cost of increasing the transmission coefficient of the entire chain and narrowing the range of input voltages at which the output signal is more or less linear. In class A, this is gone (the alternative "balanced" version has a wider input range). But in a class B transistor amplifier, the effect of load current ripple requires a more linear sensor with a wide input voltage range.

For the first time this wonderful two-optron circuit caught my eye in Uldis's publication about the voltage converter, uldis.narod.ru, of his on-board amplifier. So, on a pair of optocouplers, a thermally compensated voltage feedback is implemented in a converter with a completely galvanically isolated input part (PWM controller) and output part (filters and load).

Application of thermally compensated optocoupler isolation in voltage converters

Simply connecting an optocoupler in series with a quenching resistor is acceptable in home equipment, but completely unacceptable on board. Due to the temperature dependence of the optocoupler transfer coefficient (it is always negative, about 0.5 - 1% per degree), the stabilization point will float indecently far. From the graph (cut from the TLP621 datasheet) it can be estimated that the transfer coefficients at -25C and +75C are related as 1:1.7 for input currents of 5..25 mA (TK 0.5-0.8% / deg) and 1:2.5 for currents below 5 mA (TC 0.7-1.5% deg). By the way, that is why the input current (LED) recommended by the manufacturer is just 5..16mA - the drift is minimal.

Application of thermally compensated optocoupler isolation in voltage converters

The thermally compensated circuit reduces the TC of the entire circuit due to the fact that the second optocoupler (in Figure A1) steals the current of the primary circuit, and the share of the stolen floats with the same TC as that of the primary optocoupler (in Figure A2). Assuming the transfer coefficients A1, A2 equal to K (quite acceptable), the current attenuation coefficient of the emitter follower D = R3 / R2, we solve the simplest equation and obtain the ratio of the current through R2 (output) to the current of the input LED.

By substituting the temperature dependence K=K0-B(T-T0), where K0 is the value at T0=+25C, B is the temperature coefficient, you can solve the equation for temperature and find the optimal coefficient of the output divider D. In the normal range of changes B (0.5- 1.5%/deg), the optimal coefficient D is approximately equal to the square of K0. The control error at the edges of the temperature range decreases with increasing K0. In general, it is realistic to reduce the drift of the transmission coefficient of the entire NFB circuit by a factor of five compared to an uncompensated divider.

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K0 = 1, B=0.5% : D=1..1.5 (regulation error in the range -25..+75С no more than 7%)

K0 = 0.5, B=0.5% : D=0.25..0.4 (control error 4%)

K0 = 1, V=1% : D=1..1.2 (control error 12%)

K0 = 2, V=1% : D=4.4 (control error 10%)

K0 = 4, V=1% : D=17 (control error 10%)

Calculation of ratings for Uin = 250V, controller reference level Uref = 5.0V, K0 = 3. We set the input circuit current to the minimum possible (5 mA), then R1 = 250V/5mA > 47kΩ. Power dissipated R1 at 25% overvoltage P=(300V)^2/47k=1.9W. Choose D=3^2=9 (then X = 1.5). The current through R2 is X*Iin=7.5mA, the voltage across R2 is Uref+Ube=5.0+0.6=5.6V, R2=5.6V/7.5mA > 750 Ohm. R3 = DR2 > 6.8 kOhm. The total current consumption of the secondary circuit from the +12V battery is 8.5 mA. The maximum power dissipated by the active device falls on transistor A1 and is equal to 7.5mA * (12V-5.6V) > 50 mW (everything is normal).

Author: Uldis; Publication: uldis.narod.ru

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