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VIPER12 Datasheet(PDF) 9 Page - STMicroelectronics |
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VIPER12 Datasheet(HTML) 9 Page - STMicroelectronics |
9 / 15 page VIPer12ADIP / VIPer12AS 9/15 FEEDBACK PIN PRINCIPLE OF OPERATION A feedback pin controls the operation of the device. Unlike conventional PWM control circuits which use a voltage input (the inverted input of an operational amplifier), the FB pin is sensitive to current. Figure 9 presents the internal current mode structure. The Power MOSFET delivers a sense current Is which is proportional to the main current Id. R2 receives this current and the current coming from the FB pin. The voltage across R2 is then compared to a fixed reference voltage of about 0.23 V. The MOSFET is switched off when the following equation is reached: By extracting IS: Using the current sense ratio of the MOSFET GID : The current limitation is obtained with the FB pin shorted to ground (VFB = 0 V). This leads to a negative current sourced by this pin, and expressed by: By reporting this expression in the previous one, it is possible to obtain the drain current limitation IDlim: In a real application, the FB pin is driven with an optocoupler as shown on figure 9 which acts as a pull up. So, it is not possible to really short this pin to ground and the above drain current value is not achievable. Nevertheless, the capacitor C is averaging the voltage on the FB pin, and when the optocoupler is off (start up or short circuit), it can be assumed that the corresponding voltage is very close to 0 V. For low drain currents, the formula (1) is valid as long as IFB satisfies IFB< IFBsd, where IFBsd is an internal threshold of the VIPer12A. If IFB exceeds this threshold the device will stop switching. This is represented on figure 4, and IFBsd value is specified in the PWM COMPARATOR SECTION. Actually, as soon as the drain current is about 12% of Idlim, that is to say 50 mA, the device will enter a burst mode operation by missing switching cycles. This is especially important when the converter is lightly loaded. It is then possible to build the total DC transfer function between ID and IFB as shown on figure 10. This figure also takes into account the internal blanking time and its associated minimum turn on time. This imposes a minimum drain current under which the device is no more able to control it in a linear way. This drain current depends on the primary inductance value of the transformer and the input voltage. Two cases may occur, depending on the value of this current versus the fixed 50 mA value, as described above. START UP SEQUENCE This device includes a high voltage start up current source connected on the drain of the device. As soon as a voltage is applied on the input of the converter, this start up current source is activated as long as VDD is lower than VDDon. When reaching VDDon, the start up current source is switched off and the device begins to operate by turning on and off its main power MOSFET. As the FB pin does not receive any current from the optocoupler, the device operates at full current capacity and the output voltage rises until reaching Figure 9 : Internal Current Control Structure 60kHz OSCILLATOR PWM LATCH S Q R 0.23V Id DRAIN SOURCE FB R1 R2 C +Vdd Secondary feedback I FB Is 1 k Ω 230 Ω R 2 I S I FB + () ⋅ 0.23V = I S 0.23V R 2 -------------- I FB – = I D G ID I S ⋅ G ID 0.23V R 2 -------------- I FB – ⋅ == I FB 0.23V R 1 -------------- – = I Dl im G ID 0.23V 1 R 2 ------ 1 R 1 ------ + ⋅⋅ = Figure 10 : IFB Transfer function IFBsd IDlim IFB t ONmin V 2 ⋅ IN L --------------------------------------- t ONmin V 1 ⋅ IN L --------------------------------------- 50mA IDpeak 0 Part masked by the IFBsd threshold |
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