Cuk Converter （4）

Published:2011/8/10 22:08:00 Author:Li xiao na From:SeekIC

By Georg Gerads

Construction and alignment

A printed circuit board for a DC voltage converter in this power range requires a carefully designed layout (Figure 6), which usually has to be double-sided due to the necessary ground-plane area. The measurement amplifiers, control circuitry and power circuitry are clearly separated from each other, and ’sensitive’ connections are keep a short as possible and as broad as necessary. The large ground planes protect the measurement and control circuitry against undesirable electronic interference.



Before starting to fit the components to the board, place inductor TR1 aside. Since it will only be fitted during the alignment process. You should fit feedback capacitors C9 and C11for IC2c and IC2b. but immediately short them out by connecting short wire bridges across them (on the component side). Next you have to decide whether the negative auxiliary voltage should be generated by the transformer or the on-board charge pump. Fit the corresponding components, as well as all of the control electronics. Potentiometers PI and P2 are connected using pin headers, wired such that they have minimum resistance when adjusted fully counter-clockwise. The LEDs can later be relocated to the front panel using pin headers as well. The ICs can be fitted in sockets. Pay attention to the correct orientation of the numerous diodes, the ICs and the small electrolytic capacitors. Fitting the components for the power section, including the AMP screw terminals for the input and output voltages, should not present any problems. Bend the leads of the power semiconductors so they can later be attached to the heat sink.

At this stage, it is already possible to perform initial testing and alignment after a thorough visual examination. Connect a laboratory power supply (35 V. with the current limiting set to several tens of milliamperes) to the input terminals, and connect the two potentiometers to the pin headers. Now adjust the gain of the current control amplifier (using P3) and the voltage control amplifier (using P4) such that the voltage measured at pin 1 of the 3526 can be continuously adjusted from 0 V to 5 V using PI or P2, respectively. After this, turn P2 fully clockwise and PI fully counter-clockwise. Using an oscilloscope, check that the MOSFETs are being properly driven with a PVVM signal having a frequency of approximately 40 kHz (as measured on R1 and R2). The duty cycle of this signal can be adjusted over the range of 0-90% by rotating the current-limit potentiometer (P1).

The control amplifiers cannot presently regulate the loop, since capacitors C9 and C11 are shorted out and the loop is open because the inductor has not yet been fitted. As a result, the duty cycle can be directly adjusted using the potentiometers. Only something that can be controlled can also regulate a controller.

Now it’s time to fit the inductor and attach the fully assembled board to the heat sink in ’piggy-back’ fashion, as shown in the title photo for this article. Using eight previously drilled and tapped holes in the heat sink. As the drain and cathode voltages of the power semiconductors are present on their cooling tabs, these components must of course be properly insulated from the heat sink using the standard methods.

This is a good time to fit the assembled module into a suitable enclosure along with its power supply, and then wire everything together except the mains transformer. Instead of using the transformer, you should first operate the circuit from a laboratory power supply with current limiting. Restricting the power reduces the risk of destroying any components if there is an assembly error.

The ’wiring and connectors used for the input and output must be able to handle the amount of power drawn or supplied by the Cuk Converter. Don’t forget to turn PI fully counter-clockwise and P2 fully clockwise (100-V setting)! Nov; we come to the serious work. Connect a hefty power resistor, an incandescent lamp or a halogen floodlight to the output to provide an output load, and connect a voltmeter to the output of ICld. The output of IC2b will remain ’stuck’ and cannot affect anything. Now slowly increase the load current from 0 to the maximum value by adjusting P1.

Next, perform the above procedure the other way around: first rotate PI fully clockwise (5-A setting), and then use the voltage control (P2) to slowly increase the output voltage to its maximum value of 100 V. Naturally, a current of 5 A can only be achieved if the load resistance is not more than 20Ω (100 V - 5 A), and 100 V can only be achieved if the load resistance is 20Ω or more.

However, you’re not finished yet, since only part of the circuit’s control function is operating. Adjust both potentiometers to their minimum settings, and then cut away both wire bridges to enable operation with the full closed-loop control characteristic. Now slowly increase the setting of PI again. If the inductor starts to make squealing noises, slightly reduce the gain of the control amplifier by adjusting P3 (the maximum current will still be 5 A). Finally, repeat this procedure for the voltage control stage.

Warning: The circuit generates dangerous voltages. No part must be touched when the circuit is in operation and all relevant electrical safety precautions should be observed.

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