TDA16888-150W

TDA16888-150W_INFINEON

TDA16888-150W_INFINEON

TDA 16888: Multioutput Single TransistorForward Converter 150W / 100kHz

150W Output: 5V/18A; 12V/4A; -12V/1A; Standby: 5V/100mA

Operating Frequency: 100kHz

Contents:

1.Circuit description...................................................................

2. Circuit diagram.......................................................................

3. Test results.............................................................................

4. Bill of materials......................................................................

5. Control PCB layout................................................................

6. Power PCB layout..................................................................

7. Transformer design...............................................................

8. Smoothing choke design......................................................

9. Boost inductor design...........................................................

10. RFI choke design.................................................................

11. Picture of testboard............................................................. 3 6 7 811121314151617

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TDA16888-150W_INFINEON

TDA 16888: Multioutput Single TransistorForward Converter 150W / 100kHz

Circuit description

Multioutput single transistor forward converter with boost PFC preconverter

The power supply for a PC using up to 150 W of power must, for example, provide the followingvoltages: 5V/18A; 12V/4A; -12V/0.7A; -5V/0,3A; and Standby 5V/0.1A. If any of the mains systemsin use around the world may be used as a power source, then an input voltage range of 90V to 270VAC is required, taking into account the relevant tolerances.

The application circuit using the TDA 16888 is able to cover universal input voltage range mentionedabove (if required). In this circuit a boost converter (Q1, L2, D5, C3) works as an active harmonic filterto provide power factor correction, intermediate circuit voltage stabilization and to supply the primaryand secondary side control logic in normal and standby operation. In addition, the bridging time fordips in the mains voltage is not dependent on the mains voltage level. How the IC works in a typicalapplication circuit is described below.

Start up

When the mains voltage is switched on, the smoothing capacitor C3 is charged by a current pulse, thecurrent being limited by the winding resistances of the chokes and NTC resistor R36. Start-up

capacitor C11A is charged by a low current (<1mA) through resistors R2 and R43. Once the switch-onthreshold (14V) is reached at pin 19, the TDA 16888 changes from the passive to the active state. Inthe passive state the IC draws a maximum current of 100µA while monitoring the switching thresholdsand actively maintaining the driver outputs at the L-level. In the active state the chip first checkswhether the intermediate circuit voltage at pin 19 lies between 20% and 120% of its nominal value(e.g. between 80V and 480V for a nominal value of 400V). If this is the case, the chip concludes thatmonitoring for the intermediate circuit voltage is connected (FMEA) and there is no risk to operationfrom an overvoltage.

If the intermediate circuit voltage is found to be satisfactory, and no excess current is detected at pin6, then the PFC converter starts working at half the rated frequency to cut the IC current consumption.During this process the TDA 16888 is initially powered from the start-up capacitor C11A until the boostconverter starts to supply it, or, should the IC switch-off threshold (11V) be reached first, it switchesinto the passive state and a new start-up attempt is initiated. As soon as the intermediate circuitvoltage has reached 80% of its nominal value, the PWM also starts running, with both convertersections now operating at the rated frequency. A soft-start procedure is used for the PMW converter,the rise time being set using C14 at pin 13.

If the voltage at pin 13 is less than 0.4V, the chip interprets this condition as standby mode, and shutsoff the PWM section. In standby mode the PFC converter again works at half the rated frequency toreduce current consumption.

While the MOSFET Q1is switching, a modulated AC voltage appears at the secondary windings ofchoke L2. The voltage across the main winding of choke L2 varies during rated operation from 400Vwhen Q1 is cut off and the AC input voltage passes through zero, to 400V when Q1 is conducting andthe maximum input voltage is at its peak value. The lowest voltage across the main winding (±200V)arises when the input voltage is exactly half as large as the intermediate circuit voltage. This is whythe standby and IC supply voltages are derived from bridge rectification of the auxiliary windings onL2, in order to use both the cut-off and conduction phase of the inductance. The voltage regulators IC5and Q3/D11 are required because of the variation by a factor of 2 in the dc voltage obtained.

When the PFC converter is run up, the intermediate circuit voltage overshoots. Under low load it takesa considerable time to return to its nominal level, because of the slow discharge of the smoothingcapacitor C3. During this period the voltage regulator would cut off the MOSFET Q1(up to more than100ms), which would prevent the control logic being supplied from the boost converter choke. This iswhy the TDA 16888 has a further control loop, using input pin 20, in addition to the two control loopsfor the intermediate circuit voltage and the input current. A second output path from the boostconverter (D6, C4, R2) is taken via potential divider R1, R27 to detect whether Q1 is operating.

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TDA 16888: Multioutput Single TransistorForward Converter 150W / 100kHz

This works by the voltage at C4 being set using voltage divider R1, R27 to the nominal value of theintermediate circuit voltage, or a few percent below. The short time constant of the second output pathfrom the boost converter (C4≈10nF, R2≈500k?) means that any drop in the voltage at C4 is rapidlydetected, and the MOSFET Q1 switched on via the PFC current regulator. If necessary this is donewith such short pulses that the voltage at the main output from the boost converter does not rise, evenwith no load at all. The limited controllability of the boost converter during no-load conditions meansthat the power available at the standby output is limited. For example 5V/100mA is provided with aquite good efficiency.

Normal operation

The oscillator frequency is set by just one external resistor R24. The ramp voltages for the oscillatorand the pulse width modulation (PWM) of the PFC section are generated across integrated capacitors.The duty cycle of the PFC section varies from 0 to over 90%, and for the PWM section from 0 to 50%maximum. During one switching period, the PFC MOSFET Q1 switches on first. The PWM MOSFETQ2 switches on half a period later. For greater reliability, flip-flops are used to control this timingsequence. The oscillator therefore runs at twice the rate of the external operating frequency. Byintegrating the capacitors, external circuitry is not required, and so the current consumption is reducedbecause of the smaller capacitances.

The waveform of the rectified, unsmoothed mains voltage is detected across resistors R4A, R4B, andapplied to the first input of an integrated multiplier. The output voltage from the PFC voltage controlamplifier is taken to at the second input to the multiplier. The current at the output of the multiplier, pin4, is a reference value having the waveform of the rectified mains voltage and an amplitude controlledby the voltage regulator. The PFC current regulator controls the rectified mains current such that thevoltage drop across shunt R6 assumes exactly the same value as the voltage drop across R5produced by the output current from the multiplier. The output current from the PFC current regulator(pin 3) sets up the duty cycle for the MOSFET Q1 by comparison with an internally generated rampvoltage.

The intermediate circuit voltage is regulated by the PFC voltage regulator (pin 17, pin 18) at a levelthat is greater than or equal to the peak value of the maximum input voltage (270V?√2= 382V). Anintermediate circuit voltage of 380V is often chosen, because one must expect a maximum voltage ofthis magnitude even without a PFC converter. A 450V type smoothing capacitor (C3) is used,however, to ensure that even under transient conditions, the voltage remains below the permittedcapacitor voltage. In this case it is worth increasing the intermediate circuit voltage to 410V, and todesign the onset threshold for overvoltage limiting to be 430V (R11, R12 to give 5.5V at pin 19). Theintermediate circuit voltage still remains below 450V during transient conditions, and the benefit lies inthe 40% higher hold time, which can be bridged if the mains drops out.

The PWM converter is designed to work as a single-ended forward converter. The turn-on time isdetermined by the oscillator, as soon as the voltage at the soft start input pin 13 and the control inputpin 14 exceeds 0.4V. After the soft start phase, the voltage at control input pin 14 together with theramp voltage at pin 15 controls the turn-off time. An improved current ramp control technique (currentmode control) is used here, where the ramp voltage has an amplitude 5 times higher than thosetraditionally used. The current in the Q2 source path is also measured across shunt resistor R15, anddetected at pin 11. At a voltage of 1V at pin 11, the integrated overcurrent comparator switches offMOSFET Q2. The signal at pin 11 is also amplified by a factor of 5 by a linear amplifier, and taken viaan internal 10k resistor to pin 15. A base ramp voltage with an amplitude of 1.5V is produced acrosscapacitor C13 connected to this pin, even when there is no transistor current (slope compensation).This voltage can rise to over 6V when the maximum current flows through Q2. This allows pulse widthmodulation at higher signal levels, enabling stable operation right up to no-load conditions.

The transformer Tr1 in the forward converter works at the pre-regulated intermediate circuit voltage.This means that a higher transformer ratio can be selected, reducing the current load for theMOSFET. Furthermore, a larger duty cycle can be set during normal operation because there is asmaller variation range for the input voltage. The magnetization energy can be fed back into thesmoothing capacitor using a demagnetization winding and demagnetization diode D7.

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TDA 16888: Multioutput Single TransistorForward Converter 150W / 100kHz

In forward converters with universal input voltage range this is not possible without pre-regulation, orcauses significant losses in the required demagnetization network. In our application circuit,

overvoltage peaks across MOSFET Q2 and diode D7 are efficiently limited using diodes D22 and D27and network C31, R40. On the secondary side there is one rectifying and one freewheeling diode (D8,D9; D20, D21) for each output voltage. A damping network (e.g. R41, C29; R42, C30) is connected inparallel with every diode, to attenuate commutation-induced overvoltage spikes and transients.Another RC element (R47, C34) damps the voltage decay in intermittent dc flow, to avoid irregularpremagnetization of the smoothing choke. The output voltages are taken via separate windings on acommon smoothing choke L3 to convert the switched voltages into a flow of current. The smoothingcapacitors (e.g. C15, C28) reduce the ripple on the output voltage and serve as a storage device whenthe load changes rapidly. For high output currents it is advisable to divide up the smoothing

capacitance with small inductances (L5, L6), to compensate for the equivalent series resistance (ESR)of the capacitors.

The output voltage with the highest stability specification is controlled directly by regulating amplifierIC2 and optocoupler IC3. The other output voltages are stabilized indirectly by the choice of windingratio for transformer Tr1 and choke L3 according to the ratio of the output voltages. An extra

comparator IC6 and optocoupler IC4 monitor the standby output voltage. When the standby switch isclosed, the PWM converter is only started once the PFC converter has produced at least 90% of thenominal voltage at the standby output. Standby operation can be initiated by opening the switch.Transistor Q4 with low-pass filter at its base, prevents noise spikes and leakage currents from theoptocoupler from discharging the soft-start capacitor during normal operation.

Protective features

When a short circuit occurs on the output side, the primary current is limited by measuring the dropacross shunt R15. In applications with several output voltages, this only works as a short-termprotective measure, because the output rectifier, for example, cannot withstand the overload. Extraprotection can be provided by monitoring the output voltages for a minimum value, with a delayedswitch into standby mode.

To deal with overshoots in the intermediate circuit voltage, a switch-off threshold can be set asrequired (e.g. 5% above the nominal value) using potential divider R11, R12. If this is exceeded thanthe PFC converter cuts off the power to the intermediate circuit.

When transient mains surges occur which charge the smoothing capacitor C3 to 10% above the PFCswitch-off threshold, the PWM MOSFEET Q2 is also cut-off to protect the power supply unit. If themains voltage rises still further, then varistor R30 can limit it. Using these protective devices, theapplication circuit can withstand transient mains surges of 600V and more.

Figure 1:

A 150W PC power supply

design with power factor

correction using TDA 16888

fits inside a typical PC

power supply silver box.

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