Continuous Conduction Mode for High Peak Power Quasi
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Continuous Conduction Mode for High Peak Power Quasi Resonance Flyback Power Converter ON Semiconductor CCPG Segment Application JEAN-PAUL LOUVEL, TV SYSTEM APPLICATION MANAGER, ON SEMICONDUCTOR
hen designing a switching power converter, a design engineer typically designs the thermal solution based on the maximum output power specified by the product. For example, a 50-W converter will have to include enough heat sinking capability so that it can permanently deliver 50 W right upon start-up at the lowest input line and at the highest ambient temperature. Some applications, however, do not absorb a constant power from the source. For example, a printer draws power by pulses when the print heads are active or when the paper is processed. In this particular case, the converter will not be designed to thermally handle the peak power excursions but rather a much lower average value. Today we typically see ratios of peak to nominal power up to 2:1 in many applications.
Flyback Power Converter Low power converters (Pout < 70 W) generally use a flyback topology as this provides a) good crossregulation performance in multi output applications, b) the ability to work on a wide input voltage range (90-264 Vac) without Power Factor Correction (PFC) pre-converter and c) a very low power standby mode. The transformer provides isolation from the mains and stores the energy on the primary side before dumping it on the secondary side at the switch opening event. This simple technique limits the surrounding parts with one single power MOSFET switch and one single diode for each output voltage. If this above principle is common to all flyback converters, there are multiple sub-families that we will now analyze with their respective pros and cons. The Fixed Frequency Pulse Width Modulated (PWM) flyback converter is probably the most common architecture found today. Driven by an
internal clock, the switching frequency value can be chosen so that the Electro Magnetic Interference (EMI) generated by the converter stay away from a critical frequency range. The regulation loop defines the conduction time of the power switch and controls the energy stored/transferred by the converter. This type of converter can operate in the Continuous Conduction Mode (CCM) where the energy stored in the transformer is not fully transferred to secondary-side capacitors at the end of the off-time. The transformer remains magnetized until the next cycle appears. This mode offers a trapezoidal primary current waveform (rather than a triangular shape) and provides higher output power with limited rms currents. Owing to the CCM operation, the transformer can be designed with a higher primary inductance to improve low power / standby efficiency figures. Another popular version is the Quasi Resonance (QR) type offering so-called valley switching turnon: the power MOSFET is turned on in the valley (the minimum) of the drain-source voltage. This technique provides better EMI performances compared to a hard-switching version. The switching losses are reduced however, switching frequency reduction is mandatory to pass peak power while maintaining a complete transformer demagnetization. The higher energy stored by switching cycle imposed by this lower operating frequency will require a much larger transformer size (and cost) with high peak currents in both primary MOSFET and secondary-side diode. A third type is the hysteretic converter where the power transfer at a frozen primary peak current is ensured by modulating the switching frequency to provide the requested energy (higher frequency for more power transferred). The main subject of this article is the peak power capability for a QR flyback converter, we will now analyze in detail the behavior of this type of
POWER CONVERTER converter, in particular how it copes with peak power requirements.
Detailed Behavior of QR Flyback Converters with a High Peak Power Unlike the traditional PWM converter, this converter works with a variable switching frequency to turn the MOSFET on exactly at the point where its drainsource voltage is minimum. It naturally reduces Electromagnetic Interferences and switching losses. To keep valley switching operation, in the low output power case, the on-time reduction implies a higher switching frequency which asks again for a shorter on-time (and so-on) to limit the energy transferred from the primary to the secondary side, cycle by cycle, to be below the requested limit. To limit the switching frequency excursion, new solutions have been developed where a clamp controls both the minimum on-time and the maximum switching frequency. To keep the QR operation while ensuring minimum drain-source voltage switching, an innovative valley−lockout solution has been developed. It can work down to the 4th valley and toggles to a variable frequency mode beyond calls VCO mode (Figure 1). It ensures an excellent light load / standby power performance as demonstrated by the NCP1379/1380 from ON Semiconductor. This energy transfer looks promising but unfortunately problems come up as soon as a higher peak power is needed. To increase the energy transferred cycle by cycle, the current in the primary should be larger with corresponding wider on-time. The conduction time of the secondary diode also increases forcing the controller to decrease the operating frequency so that complete demagnetization can be ensured. While the frequency is lower, the per-cycle energy should increase forcing further down the switching frequency. This “double” effect , necessary to
Fig. 1. The drain-source voltage of a Quasi Resonance flyback operated with valley switching and valley lock-out (From 1st to 4th valley and VCO mode while energy transferred reduces).
secure a complete transformer demagnetization, will force the converter to dramatically reduce the frequency to accept a peak power excursion. The transformer should therefore be designed to accept the higher energy that has to be stored cycle by cycle: an oversized transformer will be needed to support QR Flyback Converters operating with a High Peak Power. If the above behavior creates a problem at high power, it becomes a natural advantage when the secondary output is shorted to ground. In presence of a short circuit, the demagnetization will take a longer time, forcing the frequency to be very low, thus reducing the transmitted power. Safety is greatly improved owing to this operating mode. The switching frequency will go up as soon as the secondary output voltage increases again, e.g. when the short circuit is removed.
A New Solution Featuring Peak Power Excursion We want to define a new Power conversion solution that provides a higher peak power capability without oversized parts and while keeping all the advantages of a QR-operated flyback converter. a) Keep QR for both low EMI and lower switching losses benefits at nominal/average power b) Use CCM in high power to avoid frequency reduction and larger/oversized transformer c) Keep QR Output short circuit behavior and very good natural safety performances
Higher Peak Power QR Flyback with Continuous Conduction Mode The principle of operation for the Zero Current Detection section in a NCP1380 controller appears in figure 2: With auxiliary winding voltage applied to the ZCD input, the controller is able to control the end of energy transfer. The added diode D203 and resistance R206 are used here for Over Power Compensation function of NCP1379 during the on time of the MOSFET. The circuit should be modified to allow CCM operation when a high peak power demand occurs. To avoid the complete transformer demagnetization over a defined power illustrated by a transfer time larger than a given value, an additional transistor is added. This is shown as Q206 in figure 2, whose presence forces the Zero Current Detection to re-start the controller while complete demagnetization has not yet occurred. We should also keep complete demagnetization
POWER CONVERTER GND). This new solution allows an increase of ~50% of the power capability without increasing size and cost of the overall parts (mainly the transformer) keeping all the QR nominal load and safety behavior advantages.
1 R205 1K Aux. winding 2 F R206 470k
Q206 BC848ALT1 C210 68p
CCM QR Flyback without Frontend PFC
Fig. 2. Principle of Continuous Conduction Mode Quasi Resonance Flyback
control with ZCD for average power, starting phase and output short circuit to Ground to avoid over stress and over size of multiple parts thanks to the very good natural QR safety behavior. This is done by a control of the reflected voltage on the auxiliary winding which provides a perfect image of secondary output voltage.
Detail on Added Circuit Allowing CCM for High Power Principle: Inhibition of QR ZCD allowing CCM when Power is over a given limit • C231 is charged with negative voltage proportional to supply voltage during the conduction of the primary switch • R235 and R254 combined with C231 set the delay time T and transistor Q206 is the switch to turn the circuit ON (simple timer) • Over defined time T, Q206 is switched ON and pulls the ZCD pin 1 to GND (To restart the next cycle with energy stored in the transformer). The serial cap C238 guaranties the low voltage level of the IC input despite Vce-sat of Q206 • As Q206 is supplied by R234 connected directly to the winding, the CCM is linked to the reflected voltage from the secondary side during the conduction of the secondary diode: CCM cannot be activated if reflected / secondary output voltage is too low (starting phase or output shorted to
The power capability is reduced at low mains supply. Despite shorter off time control by the CCM timer (Reflected negative voltage proportional to supply is smaller with lower mains input), the larger on time (to get the same drain current in the MOSFET) at lower mains supply has a small impact on the switching frequency reducing the power capability. This new solution can also be used without a frontend PFC to increase the peak power for applications below the 75 W limits without PFC.
Limitation of the Proposal There are two limitations to be taken into account for this solution. CCM should not be used for high output voltage applications, as this requires a very low trr secondary diode. CCM is typically limited to low Added parts for CCM control
R253 1K 1/4W res500d
R235 3K3 1/4W
R254 3K3 1/4W
Q206 BC848ALT1 sot23 D212 MMSD4148 sod123
D213 BAV21 do41d
D220 MMSD4148 sod123
2 D214 MMSD4148 sod123
Aux. winding C231 1n5 250V
R206 470k D203 MMSD4148
Ic200 NCP1379 1
7 C210 68p
Fig. 3. Detailed solution of Continuous Conduction Mode Quasi Resonance Flyback
POWER CONVERTER T200 SRW4549EM-XXXX 3
13 R251 33K 2W
R252 33K 2W
R200 33K 2W
C200 10n 500V
D220 MMSD4148 sod123 R253 1K 1/4W res500d
D201 MUR180 do41d
D213 BAV21 d041d
R235 3K3 1/4W
Q206 BC848ALT1 sot23
R254 3K3 1/4W D205 D214 MMSD4148 sod123 C231 1n5 250V
D212 MMSD4148 C238 100n sod123
R206 D203 470K SOD123 MMSD4148 1
2 Q204 BC808-25LT1
C218 100p 1KV
R221 0.47R 2W res1000 C225 Y 1nF F
10u 50v C236 100n
Q203 STP6NK70Z to220decalD
C217 47u 25V Hs3 Heat_Sink 1
Q208 BC858ALT1 sot23 R246 1 2
BO Ic200 NCP1379
1 T200-01 1
Fig. 4. Detailed & complete primary part of NCP1379 CCM QR Flyback SMPS TND401/D ON Semiconductor Green Point reference design
voltages applications (< 30 Vdc) naturally built with Schottky diodes (particularly true for 19 V adaptor’s or printer’s applications). CCM is difficult to use with high output current applications built with secondary synchronous rectification. The secondary Synchronous Rectification MOS should be switched OFF before a new cycle to avoid short circuit with this bidirectional switch. The very high current in the primary of the transformer will activate the primary over current limitation and stop the power supply.
Conclusion This new solution utilizing QR Flyback with high peak power provides an increased power capability of ~50% over standard QR Flybacks. Designing CCM for peak power avoids over sizing the transformer, MOS and secondary diodes. The design, now optimized for average power, can be more compact with improved low power / standby performance thanks to increases in the transformer’s inductance while keeping all advantages of a QR solution for nominal power.
This increased peak power capability for applications below 75 W (without PFC), allows reducing both size and cost of today’s QR Flyback Adaptors used for computers, games stations and printers. The solution, easily designed with a minimal number of low cost parts, has been demonstrated with the ON Semiconductor NCP1379 / 1380 which also provides valley lockout for improved low power performance. Combining advantages of both PWM (Higher peak power capability with CCM) and QR (Reduced EMI and switching losses, natural safety behaviors for output short circuit to Ground), the Continuous Conduction Mode for Quasi Resonance Flyback Power Converter is quickly becoming the preferred Flyback solution for future products with very good natural safety behavior (lower switching frequency with output short to ground). More details are available under the link: http://www.onsemi.com/pub_link/Collateral/TN D401-D.PDF