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Abstract Until now, most of the power supply for electronic products were the switching mode power supply. As a product that combined the control IC and the MOSFET, both used in the switching mode power supply, into one package, the SPS attempts to include the maximum number of external components to reduce the surrounding components as much as possible. It also strengthened its protection function.
Especially, the SPS KA3S series herein called SPS described in this note is especially appropriate for the quasi resonant power supply used mostly in color TVs and is being sold through recent production. Generally, the yback or forward method power supply used most widely in general electronic products employ the xed frequency hard switching method.
For monitors and other TV-like displays, this noise directly affects the display quality. This why the quasi resonant yback switching is required. Furthermore, this method reduces switching loss by turning on the SPS when the voltage across is minimum or zero. According input AC line voltage and load conditions, the frequency is made to vary through the SYNC circuit; this minimizes the display noise.
If the voltage of the SPS 5 pin has not reached Vsyth typ. Recently, TV manufacturers are worrying about having to meet the regulation on stand-by power consumption i. Energy star in U. The SPS executes the Burst Mode operation in which the SPS switching for a xed interval and stops and repeats this type of intermittent switching to reduce the switching loss which, in turn, reduces the standby power. This Burst Mode opertion can be implemented to satisfy the standby power regulation without having to add many components.
In this perspective, not only is the SPS for the TV well-suited for making a low cost, high performance power supply but also, using few components, to satisfy the standby power consumption regulation. Why the general yback method is inappropriate for the TV power supply In the general yback power supply, resonance is mainly generated between the transformer primary inductance and the capacitance, which exists between MOSFET drain-source, after the MOSFET turns off.
The instant the MOSFET primary switch turns on, the high voltage charged in the drain-source capacitor temporarily discharges, generating a large current spike. If it turns on at a very high voltage, a very large current spike, generated as the drain-source capacitor discharges, produces noise. The quasi resonance operation executes minimum or zero voltage switching to reduce this noise. Its secondary side is the same that of the general yback circuit.
Unlike the general yback and forward power supply which uses the RCD snubber circuit the power supply circuit with the KA3S series SPS uses the capacitor between the drain and source as the snubber circuit. TVs have many secondary side output voltages, but V used for horizontal deection and 15V low voltage used as power to sound or other signal processing ICs are the two basic ones. Namely, if the FB voltage increases so does the SPS duty; the secondary side receives more energy which increases the output voltage.
When the FB voltage decreases, the reverse occurs. The resistance of the resistors should be adjusted such that the REF pin equals 2. As reference, the constants of the 20 inch C-TV demo board circuit are attached at the end of this note. Figure 1. Start-Up Operation If the Vdc voltage divided by resistance of the start-up resistor results in current larger than the SPS start-up current, then, this start-up resistor is selected. Two start-up resistors are used to reduce the Set failure due to the error of resistor breakdown voltage and to reduce the Set failure related to the Surge.
Furthermore, it is best to use two small capacity resistors connected in series. Figure 1 describes this operation. At start-up, the VCC only needs to be maintained at 10V. VCC voltage above 23V does not operate in the transient state. Rev C, November The rst mode is the standby state, and the other is the normal operation state. By controlling the SYNC circuit time constants, Vds becomes minimum turning on the switch, thus, reduce the current spike and switching loss.
The snubber capacitor can reduce the snubber voltage spike due to the leakage inductance and this reduces the EMI. If the snubber capacitor is too large, the switching loss increases and the standby consumption power increases, but, if too small, the increasing switching noise at turn-off becomes a problem.
Though the power supply would be better explained through an equivalent circuit, which considered all the output terminals of the TV power supply, it will be explained assuming having one output for it is that way, in principle. Each part of its operation according to time is examined. Figure 3. When the MOSFET turns off, the energy stored in transformer magnetic inductance increases the equivalent output capacitor Coss voltage of Vds and the snubber capacitor voltage between the drain-source.
Because the equivalent output capacitor Coss of the MOSFET is much smaller than the snubber capacitor, most of the charging current ows to the snubber capacitor. Until Vds voltage reaches the sum of the DC input voltage Vi and primary side winding voltage Vr:nVo from to the secondary side voltage at t1 , the energy stored in the transformer magnetic inductance is not supplied to the secondary side during this interval because the secondary side diode is not on.
The nVO is how the primary side perceives VO. Here, the rectied input voltage, Vi, is the DC voltage having twice the AC line frequency and the secondary side voltage is almost a xed DC voltage.
For this reason, the Vi voltage includes twice the ripple of the AC line frequency. During the switching period, however, Vds is assumed to be most DC voltage. The turn-off time from t0 to t1 is related to the equation below. When Vds voltage reaches Vds1 at t1, the equivalent circuit Figure 4 is formed, while the diode connected to the secondary side transformer winding turns on.
The diode current begins to reduce linearly from the yback moment but ows until it becomes zero at a slope proportional to the output voltage and turn ratio n. During this interval, current does not ow in the primary side. If this interval time, t1 to t2, is actually calculated, it can be obtained from the equation below. This circuit mainly determines when the switch should turn on in the next period. The VCC winding voltage waveform is determined according to the transformer turn ratio.
As the SYNC circuit connected to 5 half-wave recties the VCC winding voltage using diode IN , 5 pin voltage increases linearly when the secondary diode turns on and drops when the diode turns off. The circuit below shows the charging path when the sync pin voltage increases because of a turned-on diode. In circuit 5, the larger the R3 and C2, the lower the voltage rising slope.
The SYNC voltage is limited to 8. The following equation calculates this voltage. Figure 4. Figure 5. Charging Circuit when the Secondary side Diode Turns On The resistor and capacitor related to this circuit should be designed carefully such that Sync voltage does not exceed Vsyth in the standby state but rises to about 8V during normal operation and that minimum voltage Vds switching is veried in normal operation. A capacitor of lower than about few hundreds pF satises the above conditions.
In this case, the Vds voltage can be divided mainly into two parts. It is the sum of the primary side input voltage Vin and nVO. The applied transformer primary side voltage n turn ratio times the output voltage VO when the diode has turned on. The secondary side voltage does not affect primary MOSFET voltage any more and, from this point, the charged energy between the output equivalent capacitor Coss and the snubber capacitor Cr starts to resonate through the primary transformer inductor Lm.
The resonance voltage and current shape in this case have cosine and sine waveforms, respectively. After passing t3, the inductor voltage polarity reverses, making the current increase. As Vlm increases, the capacitor voltage Vds gradually reduces and its minimum value can be divided into three cases. In this case, minimum Vds is Vin-nVO. Because nVO equals Vin, the minimum is zero.
Zero voltage switching occurs; MOSFET turn-on loss becomes zero; and noise due to the current spike at turn-on is not generated. As shown by the equation above, the resonance interval is proportional to the transformer inductance and resonance capacitor Crs , the sum of the MOSFET output capacitor and snubber capacitor. Vsync is calculated by the equation below. The SYNC equivalent circuit of this case is shown below.
If the transformer inductor value changes or snubber capacitor value is changed, this time setting must be veried. In this case, the minimum Vds is Vin-nVO.
Of course, the MOSFET turn-on loss becomes zero because of zero voltage switching; and the noise resulting from the current spike at turn-on is not generated. The charged snubber capacitor discharged as SPS turns on. At turn-off, the snubber capacitor much larger that the MOSFET output equivalent capacitor Coss reduces the Vds rising slope, which reduces the noise generation. If the input voltage is varies, the power supply will operate through the above three modes.
If the AC line input voltage is low, it will operate through mode 1 or 2 and, if high, through mode 3. The equivalent circuit in this interval is shown above. The energy stored in the transformer internal inductor is proportional to this current. This energy is 0. Because the energy in the inductor is proportional to the square of the current magnitude, the bigger the output load, the higher the inductor current.
The lower the input voltage and larger the feedback voltage i. The higher the input voltage and lower the output load, the higher the switching frequency becomes. The output load in the standby mode is very small.
Using this method in the standby state greatly reduces the switching loss but is limited when trying to satisfy the recent consumption power regulation of the standby state. Currently, using a auxiliary-power supply or changing the other secondary circuits has become common in trying to meet this requirement, but these methods have imposed the C-TV manufacturers because of their additional costs and components.
Although the design values are difcult to develop as equations through the SPS method, it can be derived in general form because the parameters of each component have been determined.
This method is a well-suited for meeting the standby state consumption power regulation without adding many components. Swiching frequency vs line voltage 80 Switching frequency kHz 70 60 50 40 30 20 10 0 80 Freq. Burst Mode Operation A. Introduction Recently, many countries and regulations are requiring the Stand-By Power to be lower than a set wattage. Although various other method including the auxiliary power supply have been adopted by Set manufacturers, the SPS with the Burst Mode Operation can be chosen to effectively meet this regulation.
3S0680RF Original Pulled Fairchild Integrated Circuit Business & Industrial
Abstract Until now, most of the power supply for electronic products were the switching mode power supply. As a product that combined the control IC and the MOSFET, both used in the switching mode power supply, into one package, the SPS attempts to include the maximum number of external components to reduce the surrounding components as much as possible. It also strengthened its protection function. Especially, the SPS KA3S series herein called SPS described in this note is especially appropriate for the quasi resonant power supply used mostly in color TVs and is being sold through recent production. Generally, the yback or forward method power supply used most widely in general electronic products employ the xed frequency hard switching method. For monitors and other TV-like displays, this noise directly affects the display quality. This why the quasi resonant yback switching is required.
Jest uszkodzony. It has a basic platform well suited for cost effective C-TV power supply. Wide operatimg frequency range up to kHz Pulse by pulse over current limiting Over load protection Over voltage protecton Min. DRAIN 2. GND 3. VCC 4.