linkswitch-ph constant voltage

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is it possibleto use the linkswitch-ph family for a constant voltage LED PSU?
we are after a low cost, high efficiecny PFC power supply
output ripple can be quite high (3Vpp)
power 20W
woltage 24V
efficiency >90%

regards

we haven't done full evaluation of operating linkswitch-ph in CV applications.

I have modified a RD-194 demo board for constant voltage operation.
The modification is: (reference figure 6 LNK406 data sheet)

1. Add a 15K resistor in series with the emitter of Q3 to reduce the feedback circuit gain.
2. Change the zener VR3 to adjust the desired output voltage. I put a 20V zener diode in parallel with VR3 to adjust the output voltage to 21 volts.
3. The capacitance on the power output is 2,000uf in addition to the 660uf on the RD-194 demo board.

The voltage is stable for output currents ranging from 50ma to 492ma current limit.
Load currents less than 50ma cause the LNK406 to go into burst mode but the output voltage is stable with the large output capacitance.
The LNK406 operates continuously when the load current is increased to the 50-492ma region.

What I want to know is how to extend the continuous operating load current to a lower value.

Richard Keller 3/26/2012

The burst-mode you're referring is the autorestart function of the IC when the IFB current reaches IFBAR which is basically what youre doing when you reduce the output power.Pls see Figure 5 on pp 3 of the datasheet.

One suggestion if acceptable with your design is to put a 50mA preload or active preload (for better efficiency at full load condition) on the output so you can operate at less than 50mA without entering the autorestart.

I have noticed that increasing the current into the "V" pin will decrease the programmed output current.
Does this affect the DC Max. in Figure 4 on pp3 of datasheet.
Increased "V" pin current decreases the output current.
Could a zener diode between D7 cathode and "V" pin be used to regulate the output voltage?
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Richard Keller

You can use the Vpin as long as you operate between its boundaries, OV and UV limit. Be sure also to check the compromise on OV and UV performance.

I can see the limitations on using the V-pin.
I tried the PWM control using the FB pin, it works well.
Especially when running just below the switching frequency.

What would the lowest cost, voltage to frequency converter circuit, that works well with this ic? Possibly a 555 circuit?

Richard Keller

I'm not sure what you mean by PWM control on FB-pin while maintaining a CV and operate at very low output current without hitting the auto restart which is the original problem. Can you expand more on it?

Digital PWM Dimming as described in the PI Link.
http://www.powerint.com/forum/power-supply-design/can-i-perform-pwm-dimming-using-linkswitch-ph
This document described "Digital Dimming" by closing a switch between the BP pin and the FB pin. I actually used a pulse generator with a coupling diode to pull the FB pin high(above its threshold). The pulse generator uses a constant pulse width with variable frequency to adjust duty cycle. This works great.
The output voltage is constant so the ic is powered correctly. The switching transistor is inhibited when the FB pin is high(switched to BP)and operating normally when the FB pin is not driven by the switch.
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In my test the FB pin is driven high for 10us or more for duty cycle greater than 50% (low output current). The FB pin is not driven for 10us or more for duty cycle less than 50% (higher output current).
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I do not think it is going into auto restart. The high voltage switch is not inactive for ~1.3seconds.
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The maximum inhibit time in my test is 150us (switch on 150us, switch off 10us). I see the high voltage linear regulator activated when the inhibit time is 100us.
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Richard Keller

Thats great if it works ok. Are you laading it with CV or CR(constant resistance) load? Just curious if the beat frequency also appears with your current configuration.

Thats great if it works ok. Are you loading it with CV or CR(constant resistance) load? Just curious if the beat frequency also appears with your current configuration.