Trailing Edge dimmers for LED lamps have problems?

[treez]

Is it true that Trailing Edge dimmers, when used with LED lamps, give problems such as poor power factor?
Also, a trailing edge dimmer breaks the live connection at some angle in the mains cycle.
Therefore, it will be breaking an inductive current (since the supply wires contain inductance)
Thus , inside every trailing edge dimmer , there will need to be a hefty MOV to be able to quell the inductive overvoltage?

Wouldn't the following dimming scheme, involving clipping out mains half cycles at the zero cross points be a better approach to dimming?.....i.e., some pattern of 'clipped out' mains half cycles means "dim the lamp", then the controller in the lamp goes ahead and dims it....that is, the mains is kind of being used as a comms wire. A pushbutton switch in the dimmer could "chop out" a mains half cycle.

The below shows the new proposed dimmer...one is the circuit in the wall switch dimmer......the other is the led driver with circuitry to listen for dimming commands.

 

[betwixt]

The problem of back EMF isn't as bad as you think. Firstly, the inductances used in EMC filters are usually quite small, they are primarily to reduce interference below 30MHz or so. Secondly, they are usually accompanied by capacitors which themselves tend to 'round off' the abrupt voltage rise as the triac starts to conduct and thirdly, the current lags the voltage in an inductive load so you really have to worry about VA rating rather than Watts rating. Don't forget it's the leading edge of the mains cycle that the triac works on so the lamp itself is it's own snubber, the triac turns off at zero crossing anyway.

The underlying problem though is that a triac costs pennies and your control circuit would cost considerably more.

Brian.

 

[treez]

I also would have thought that the filter inductances in a led lightbulb would be small....however, all the app notes show them being, quite large, as inpage 1 of the following where its 1mH...
http://www.powerint.com/sites/default/files/product-docs/an49.pdf

I have found that simulating triac dimmers causes a severe overvoltage on the primary side DC bus of the led lightbulb when triac dimmers are used...this is due to the resonance between the emi inductor and the dc bus capacitance of the led lightbulb.
Thus i believe Triac dimmers are a big problem with led lightbulbs (as opposed to cfl) and so shouldn't really be used.

I am speaking here of "Trailing Edge" dimmers, and not Triac dimmers (which as you know, triac dimmers are leading edge).

I believe that triac dimmers are terrible for led bulbs, but also that "trailing edge" dimmers are also not very good for led lightbulbs (due to poor power factor, and the increased rms primary current due to the overall reduced voltage that one gets, on average , at the primary (mains) side, as the mains is literally being chopped.

This is why i am offering the above alternative. Do you agree that the trailing edge dimmer has the said deficiencies?....trailing edge dimmers , as you know, don't use triacs, and are more expensive circuits than triac dimmers.

 

[betwixt]

I would consider 1mH as being quite small, especially when considering 50Hz operation. The problem with trailing edge dimmers is although they reduce the power output when measured across a full cycle, they may still allow peak voltage to appear across their output. As the power supply in many lamps has a sizeable reservoir capacitor it still allows quite large current spikes to be drawn from the AC mains. It's a trade off between higher voltage and higher current.

I would still recommend dimmable LEDs use PWM and in the case of industrial LED lighting, that multi-phase PWM is used to lessen the risk of strobing. The technology is well established and inexpensive and it gives more precise control than most other systems. My own experience of PWM dimming is that it is very reliable. Of course it shifts the burden of design into the realms of software as much as hardware but even the simplest microcontrollers can produce PWM signals and they can easily implement such things as soft start or even timer or remote control.

You might find this interesting, it is one of the better explanations of the technology: **broken link removed**

Brian.

 

[treez]

Thanks Betwixt,
Your linked article does seem to echo my view that whether leading or trailing edge dimmer is used with leds, both leave much to be desired......it says that no "dimmable" led lamp can be expected to work with all trailing edge dimmers.........and it points out that the power factor, and rms currents are considerably worse even with trailing edge dimmers.
So far, the alternative I present seems the next cheapest option.....and of course , could be adapted to work in pwm dimming mode, as you discuss