Cuk Converter （3）

Published:2011/8/10 21:44:00 Author:Li xiao na From:SeekIC

By Georg Gerads

Control loops

For monitoring the currents and voltages in the power section and driving the switching transistors, we use a type 3526 IC. which is available from several manufacturers (including TI and ST). Although- this is a special-purpose IC. it is a well-proven industry standard and thus fairly ’.veil known. Detailed information for this IC is avail¬able from the manufacturer’s data sheer, so here the block diagram of this pulse-width modulator IC (Figure 5) is sufficient for understanding how it works. The 3526 is a PWM controller for push-pull converters, so the drive signals at its outputs (OUT A and OUT B) are pulse-width modulated according to the value of the control variable. The output signals have a maximum duty cycle of 50:; less the dead time, with a phase offset of 180 degrees. Diodes DiO and Dll combine the two output signals. This yields a PWM signal with a duty cycle ranging from 0 to 100%, less two dead-time intervals.



The internal output drivers obtain their operating voltage via VC (pin 14). As can be seen, R27 limits the current through these transistors to prevent them from becoming excessively saturated, so they won’t generate undesirable current spikes during switching transients (both transistors "on-). The internal transistors don’t require a lot of current, since they only have to provide the base currents for a pair of external driver transistors (T3 and T4), which in turn drive the power MOSFETs (Tl and T2).

The period of the oscillator is set to just under 20 us (equivalent to 50 kHz) by C17 and R28. while R29 sets the dead time to 6 us. The internal PWM latch is clocked by the oscillator, but it is also affected by the error amplifier (-ERR on pin 1) and the current sense inputs (-CS and -CS). The -ERR input is connected to COM? so the error amplifier acts as a voltage follower That’s all we need here, since external opamps (iCl and IC2) are used to amplify and condition the two measured variables U™ and IQ_ to suit the requirements of the 3526. This isn’t all that simple, since the measured quantities are negative with respect to ground and must be inverted before they can be used by the PWM controller IC.

The reference voltage output VREF (pin 18), which provides exactly -5 V. is used for the signal conditioning circuitry. Opamps ICld and ICla invert the reference voltage and allow set-point values in the range of 0 to -5 V to be set fay adjusting Pi and P2. The measured quantities, which represent the actual values, are amplified by a factor of -10 by IClc (for the output current) and IClb (for the output voltage) and summed with the set-point voltages. Sense resistor RS and voltage divider R4-R7 are dimensioned such that the voltage at the output of 1C lb or IClc is -5 V for an output current of 5 A or an output voir age of 100 V. respectively.

Each circuit has a control amplifier (IC2b or IC2c) at its output, and the output voltage of the control amplifier is regulated such that the sum of the negative set-point value and the positive actual value is exactly zero. The two signals are ORed via D5 and D6. which causes the lower of the two voltages to reach the input to the error amplifier inside the 3526 and thus determine the duty cycle of the PWM modulator.

The 3526 input is connected via R20 to — 5 V. which corresponds to the maid-mum duty cycle. The lowest voltage on the control amplifier outputs thus always dominates the control loop. The duty cycle always adjusts to meet the demands of whichever control amplifier is effectively ’in the loop’. The two remaining opamps (IC2a and IC2d) are used as comparators to visually indicate when the upper voltage limit or upper current limit is reached. They allow the user to see whether the circuit is operating under voltage control or current control.

The measured quantity for the input current (imax) is easier to handle. Here the 3526 provides an internal Schmitt-trigger comparator with a 100-mV fixed reference voltage. If the measured voltage reaches this value (after being divided by R30/R31 and smoothed somewhat by C18), the comparator triggers a shutdown. The PWM modulator will not start up again until the measured voltage drops below 80 mV.

Finally, a soft-start function is implemented using R21. C14 and IC4b, and manual reset capability is provided by R22, SI and IC4a. The outputs of these two comparators are ORed together and connected to the Reset and Shutdown inputs of the 3526. Connecting theses two pins together causes the IC to execute a soft start after each overcurrent event.

A thermostatic switch (normally open) can be connected in parallel with SI to monitor the temperature of the heat sink. A PTC thermistor could be used for the same purpose. Thermistors have extremely steep characteristic curves. For a type with a rated temperature of 60℃, the resistance rises from a few ohms to the megaohm range when the temperature increases from 50℃ to 70℃. This means that any type with a rated temperature of 50-70℃ is suitable, such as the B59901-D60-A40 or any other type with a ’50’. "60’ or ’70’ in the middle of the type number.

The 3526 has internal undervoltage protection and overtemperature protection, which, also trigger a shutdown.

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