The following questions are compiled from various customers. The answers given might be applicable to the specific application but not others.
| What is the difference between using a R-C snubber versus a peal-clamp device across the primary winding, to protect the VIPer? |
R-C SNUBBER CLAMPS THE INITIAL PORTION OF THE SPIKE BASED ON ITS TIME CONSTANT, IRRESPECTIVE OF THE MAGNITUDE AND DURATION OF THE SPIKE. SO USUALLY IT REQUIRES A HIGHER DESIGN MARGIN. ALSO R-C SNUBBERS DISSIPATE ALMOST CONSTANT AMOUNT OF POWER EVERY CYCLE, IRRESPECTIVE OF THE O/P LOAD, WHICH MAKES THEM LESS SUITABLE FOR POWER SUPPLIES WHERE STANDBY POWER CONSUMPTION NEEDS TO BE LESS. ON THE OTHER HAND, PEAK-CLAMP DEVICES, SUCH AS PKC-136 FROM ST, CLAMPS THE SPIKE TO A VERY PRECISE VALUE, BUT THEY COST MORE.
| The output voltage of my VIPer power supply droops after a certain load level. What are the possible reasons? |
A) POWER LIMIT BY MEANS OF CYCLE BY CYCLE CURRENT LIMITING OF VIPER MAY BE TERMINATING THE SWITCHING CYCLES PREMATURELY. IT CAN BE CONFIRMED BY SEEING THAT, VARYING THE INPUT VOLTAGE VARIES THE O/P LOAD AT WHICH O/P START DROOPING. THIS REQUIRES A CHANGE IN THE TRANSFORMER PRIMARY.
B) THE VDD SUPPLY SHOULD BE CHECKED TO ENSURE THAT IT IS OPTIMIZED FOR ALL LINE AND LOAD CONDITIONS.
| VIPer runs very hot on my test bench, even at a very low o/p power. What could be the possible reasons? |
A) VIPer HAS A HIGH VOLTAGE START-UP CIRCUIT INTERNAL TO IT WHICH IS DISSIPATIVE. IDEALLY IT WORKS ONLY THE FIRST TIME WHEN VIPer CIRCUIT IS TURNED ON. HOWEVER, SOMETIMES WHEN THE VDD SUPPLY IS NOT OPTIMIZED, IT CAUSES THE HIGH VOLTAGE START-UP CIRCUIT, INTERNAL TO VIPer, TO CONDUCT PERIODICALLY, WHICH CAN BE VERIFIED BY SEEING A SAW TOOTH ON THE VDD PIN.
B) THE CURRENT FLOWING THROUGH THE VIPer (DRAIN TO SOURCE), SHOULD BE VERIFIED TO ENSURE THAT THE TRANSFORMER IS NOT SATURATING.
| How to center VDD voltage? |
FOR VIPer20, 50, 100, SIZE THE VDD RESISTOR TO CENTER THE VDD VOLTAGE WHEN THE LOAD CHANGES FROM MIN TO MAX.
| What kind of snubber to use? |
FOR LOW POWER (~5W), USE R/C; FOR HIGHER POWER >/= 10W, USE R/C/D OR TRANSIL.
| How high can you run VDS? |
THE STATED VOLTAGE IN THE DATASHEET OR TO YOUR COMPANY'S DERATING POLICY.
| VIPer is not starting-up; what could be the cause? |
1) CHECK VDD
2) VDD SET POINT TOO LOW, CHECK RESISTOR VALUE AND TRANSFORMER WINDING
3) START CAPACITOR VALUE MAY BE TOO LOW
| What is the amount of copper needed for heat sink? |
LOOK AT SPECIFICATION UNDER THERMAL IMPEDANCE.
| What frequency is best to operate at? |
FOR LOW POWER, USE 60KHZ; FOR HIGHER POWER, USE 100KHZ.
| How can I improve EMI results? |
1) MAKE CURRENT LOOPS AS SMALL AS POSSIBLE WITHOUT REDUCING COPPER AMOUNT NEEDED.
2) SE SEPARATE RETURNS FOR SIGNAL AND POWER GROUNDS.
3) SE SNUBBERS ON VIPER AND/OR OUTPUT RECTIFIERS.
4) SE "Y" CAPACITOR CONNECTED BETWEEN SECONDARY AND PRIMARY.
5) COMMON MODE CHOKE IS MORE EFFECTIVE THAN DIFFERENTIAL MODE CHOKE.
| How do I choose the output capacitor? |
USE A LOW ESR TYPE, WHICH WILL MEET YOUR RIPPLE CURRENT REQUIREMENT. THE MAXIMUM RIPPLE CURRENT IS 0.9 X THE OUTPUT CURRENT. ALSO, CHOOSE THE ESR SO AS TO MEET THE NEEDED OUTPUT RIPPLE VOLTAGE REQUIREMENT.
| How can I calculate the maximum reverse voltage VrsD across the output rectifier diode of the secondary side in a flyback converter with VIPerXX? |
THE REVERSE VOLTAGE VrsD CAN BE OBTAINED BY THE FOLLOWING EQUATION:
VrsD =2*[Vout+ Vinmax* Vout/ VR]
WHERE:
Vout IS THE OUTPUT VOLTAGE
Vinmax IS THE MAXIMUM INPUT VOLTAGE
VR IS THE REFLECTED VOLTAGE OF THE TRANSFORMER
| Is there a snubber or clamp network to put on the secondary side of a flyback converter with VIPerXX0 that can allow you to use an output rectifier diode with a lower reverse voltage VrsD? |
YES THERE IS. A SERIES “RC” NETWORK CAN BE USED CONNECTED BETWEEN THE ANODE OF THE RECTIFIER DIODE “D” AND THE OUTPUT GROUND.
DOING THAT, THE REVERSE VOLTAGE ACROSS THE DIODE VrsD CAN BE OBTAINED BY THE FOLLOWING EQUATION:
VrsD =2*[Vout+ Vinmax* Vout/ VR]
AND THE VALUE OF Csn AND Rsn ARE GIVEN BY:
Csn = TWO OR THREE TIMES THE PARASITIC CAPACITANCE OF THE RECTIFIER DIODE (CD).
Rsn =2*sqrt(Llks /(Csn+ CD))
WHERE:
Llks = LEAKAGE INDUCTANCE OF THE SECONDARY SIDE OF THE TRANSFORMER
| Is there any software for the design parameters calculation of a SMPS using ST VIPer device? |
DESIGNING SMPS WITH EVERY TYPE OF VIPer IS AVAILABLE THE NEW VERSION OF VIPerXXX DESIGN SOFTWARE.
| Is the VIPerXXX Design Software available for download from Internet? |
YES, THE VIPerXXX DESIGN SOFTWARE IS AVAILABLE FOR FREE DOWNLOAD.
| In case of multiple output, secondary feedback, how is VDD being stabilized? |
IN SECONDARY REGULATION MODE ONLY THE MAIN OUTPUT VOLTAGE IS DIRECTLY REGULATED BY THE VIPer THROUGH THE "COMP" PIN INPUT. THE REST OF THE OUTPUT VOLTAGES INCLUDING THE VDD VOLTAGE IS REGULATED BY COUPLING OF THE TRANSFORMER.
| What is the effect to the output voltage if VDD changes dynamically? |
OUTPUT VOLTAGE IS ALWAYS REGULATED EVEN IF VDD CHANGES DYNAMICALLY.
VIPer IS DESIGNED WITH CURRENT MODE CONTROL TOPOLOGY IN WHICH OUTPUT IS REGULATED AGAINST OUTPUT DINAMIC LOAD AS WELL AS INPUT VOLTAGE VARIATION. HOWEVER, VIPer WILL SHUTDOWN IF VDD IS BELOW THRESHOLD VOLTAGE VDD(off). THE WORKING RANGE HAS BEEN SET HIGHER THAN VDD(Off) VALUE AS DEFINED IN VIPer DATASHEET.
| How to reduce VDD fluctuation in a typical design? |
TRANSFORMER DESIGN IS VERY IMPORTANT. SO, IT'S NECESSARY TO ENSURE THAT THE LEAKAGE INDUCTANCE IS AS LOW AS POSSIBLE (TYP. LESS THAN 10 mH). FOR INITIAL PROTOTYPE TESTING, A 12 V ZENER WILL HELP TO CLAMP VDD TO A MAXIMUM OF 12V.
| Why power supply output power is reduced by half while input voltage is varying from one value to another (wide input range)? |
THE VALUE OF VARIATION IS NOT SPECIFIED, SO THIS QUESTION WILL BE ANSWERED PARTIALLY.
IF THE POWER SUPPLY IS DESIGNED FROM 180 V TO 265 V AC, REDUCING THE INPUT VOLTAGE BELOW 180 V MAY CAUSE THE PEAK PRIMARY CURRENT TO EXCEED THE INTERNALLY CLAMPED ID VALUE (ESPECIALLY WHEN A ZENER DIODE IS CONNECTED AT THE COMP PIN TO GROUND). IN THIS CASE OUTPUT VOLTAGE WILL DROP TOGETHER WITH OUTPUT POWER.
| Can you please provide high frequency transformer design reference manual ? |
IN CASE YOU ARE STILL NOT AWARE, VIPer COMES WITH A DESIGN SOFTWARE, AMONGST OTHER SUPPORT TOOLS, WHICH HELPS THE DESIGNERS TO EASILY DESIGNED THE PERIPHERALS OF THE VIPer CIRCUIT INCLUDING THE TRANSFORMER DESIGN.
WHAT THE DESIGNER HAS TO DO IS TO FILL IN THE REQUIREMENTS LIKE SWITCHING FREQUENCY, OUTPUT POWER, INPUT VOLTAGE RANGES ETC., INTO THE SOFTWARE. THE CALCULATIONS WILL BE DONE BY IT AND THE CORRECT TRANSFORMER DESIGN WILL BE RECOMMENDED.
THE VIPerXXX DESIGN SOFTWARE IS AVAILABLE FOR FREE DOWNLOAD.
| From VIPerXX0 datasheet, it's recommended Duty cycle (D) >80% while operating frequency is 100 kHz. Is this apply to fixed AC voltage input or wide AC input voltage? |
THIS IS NOT A SPEC. IT IS ONLY A RECOMMENDATION. THIS RECOMMENDATION SHOULD BE VIEWED WITH RESPECT TO THE MAXIMUM OPERATING DUTY CYCLE, NORMALLY OCCUR AT MINIMUM INPUT VOLTAGE (IN THE PRESENT CASE 180V AC) AND MAXIMUM OUTPUT POWER. TYPICALLY IF THE MAXIMUM OPERATING DUTY CYCLE IS DESIGNED AT 50%, WE ARE SAYING THAT YOU MAY CLAMPED YOUR PEAK DUTY CYCLE (WHICH MAY OCCUR DURING SHORT CIRCUIT OR STARTUP CONDITIONS) AT 80% IF THE SWITCHING FREQUENCY IS AT 100 kHz.
| How does this D>80% being defined? |
AS MENTIONED BEFORE THIS IS ONLY A RECOMMENDATION. THE DESIGNER CAN SET HIS OWN VALUE ACCORDING TO HIS OWN EXPERIENCE.
| Is missing cycle >80% or effective duty cycle >80%? |
EFFECTIVE DUTY CYCLE.
| Can duty cycle be used to calculate the number of returns directly? |
IT IS ONLY ONE OF THE PARAMETER USED TO DESIGN THE NUMBER OF TURNS OF A TRANSFORMER. THE MAXIMUM OPERATING DUTY CYCLE IS DIRECTLY PROPORTIONAL TO THE NUMBER OF TURNS OF THE PRIMARY WINDING IN A TYPICAL FLYBACK DESIGN.
| Does VIPerXX0 has peak voltage clamp built-in at VDD pin and Source pin? |
NOT INTERNALLY. HOWEVER A CLAMP CAN BE ADDED EXTERNALLY EITHER BY USING A ZENER CLAMP OR A RCD CLAMP AT THE DRAIN PIN (NOT SOURCE PIN). PLEASE REFER TO APPLICATION NOTES. A ZENER CLAMP CAN ALSO BE ADDED AT THE VDD PIN (E.G. 12V ZENER) IF NECESSARY.
| Is it necessary to add external peak voltage clamp network components at both pins? |
FOR LOW POWER DESIGNS (LESS THAN 20W) AND IF USING LOW LEAKAGE INDUCTANCE TRANSFORMERS, OR IF USING FOR 110V MAINS, NO CLAMP IS NEEDED AT THE DRAIN PIN. OTHERWISE, REFER TO ANSWERS TO QUESTION 4.1
| Please provide reference design material regarding the above. |
REFER TO VIPerXXX DESIGN SOFTWARE.
| From manual calculation, if the capacitor Cvdd=22uF, the start up time is approximately 5.0msec using a VIPerXX0. Is this value too small? |
IF START-UP TIME IS TOO SMALL AND DESIRED TO BE LENGTHEN, SOFTSTART FUNCTION CAN BE ADDED AT COMP PIN BY INCREASING CAPACITOR VALUE OF RC NETWORK THERE.
| Will it cause start up problem? |
IT MAY, IF THE CAPACITOR IS TOO SMALL. INITIAL START-UP IS BY BUILT IN START-UP
CIRCUIT THROUGH INPUT FROM THE DRAIN PIN, ABOUT 2mA IS DRAWN, 1mA TO START-UP
THE VIPerXX0 AND THE OTHER 1mA TO CHARGE THE VDD CAPACITOR.
ONCE THE VIPer
ATTEMPT TO START-UP, ABOUT 15mA IS REQUIRED BY IT AND THIS CURRENT HAS
TO BE SUPPLIED BY THE CAPACITOR. VDD VOLTAGE WILL START TO
DROP AND IF INSUFFICIENT CHARGE IS AVAILABLE FROM THE CAP, VDD
MAY DROP BELOW 8V AND THE VIPer
WILL SHUT DOWN. THE WHOLE START-UP CYCLE WILL NEED TO BE RE-INITIATED
AGAIN. FOR OUTPUT POWERS OF 20 TO 30W, A 47uF CAP MAY BE OPTIMUM. THE
CURRENT REQUIRED BY VIPerX2A AND VIPerX3 IS VERY LOW AND THE DIMENSION
OF THE CAPACITOR IS REDUCED. PLEASE REFER
TO THE DATASHEET FOR THE VALUES.
| From a typical compensation network (figure 15), will the compensation network design C1 (Comp to ground)= 22pF, C2 (Comp to resistor)=0,1uF & R2(C2 to ground) =1k ohm cause ABNORMAL effect on VIPerXX0? |
C1, IN YOUR CASE, IS ONLY FOR NOISE FILTERING. IT CAN BE INCREASED IF NECESSARY TO 1 OR 2nF TYPICAL. C2 IS all right. R2 CAN BE HIGHER FOR FASTER RESPONSE.
| If yes, please provide material to determine the best component value? |
THE MATERIAL IS AVAILABLE ON LINE. KINDLY REFER TO IT.
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