IRPLHALO1E Datasheet IRPLHALO1E | P13-P15

IRPLHALO1E

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The IRPLHALO1E reference design board uses a toroidal output transformer that has the

secondary windings spread over as much of the core as possible to minimize the leakage

inductance, which is inherently high in such constructions. This allows sufficient current to

shut down the convertor rapidly when the output is short circuited.

Note

Ready wound toroidal output transformers as used in the IRPLHALO1E design kit are available

from Vogt or Kaschke. These have many advantages including high isolation breakdown capability

because of the moulded plastic separator between the primary and secondary.

Short circuit current

The short circuit current that appears at the primary is dependent on the primary leakage induc-

tance of the output transformer as well as the value of the half bridge capacitors. The half bridge

capacitors should be kept as small as possible in order to limit this current. However, they also need

to be large enough to be able to handle the ripple current, i.e. half the primary current flowing in

each capacitor at the IR2161 oscillator frequency.

Adaptive Dead Time

Because of the fact that the DC bus voltage varies during the mains half cycle the dead time may

need to vary in order to achieve soft switching all of the time. The IR2161 has an adaptive dead time

system that detects the point at which the voltage at the half bridge slews to 0V (COM) and sets the

LO output high at this point. There is an internal sample and hold system that allows the same delay

to be used to set HO high after LO has gone low. This reacts on a cycle-by-cycle basis of the

oscillator and therefore will adjust the dead time as necessary regardless of external conditions.

The designer does not need to take into account parasitic capacitances in the MOSFETs or leakage

inductance in the output transformer and set the dead time accordingly.

The system is designed operate down to dead times of less than 250nS which should be low

enough to accommodate the output transformer leakage inductance and parasitic MOSFET ca-

pacitances of a practical Halogen convertor. If the ADT function does not operate the dead time

reverts to a preset fixed value that will, in most cases, prevent serious losses due to hard switching.

The slew rate can easily be increased, if necessary, by increasing the value of the snubber capaci-

tor. This does, however result in the failure of the VS voltage to slew completely at reduced loads

and so CSNUB should be kept as low as possible, but high enough to maintain the supply for the

charge pump to VCC. Some of the supply current may also be provided through RD and CD allow-

ing the charge pump to provide only part of it and hence the value of CSNUB can be as low as

220pF.

IRPLHALO1E

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In the event of the voltage at the half bridge failing to slew all the way down to COM the adaptive

dead time circuit will time out after 1 to 1.5uS and switch on the relevant MOSFET thus operating

with hard switching. This could happen during the first few cycles after start up. It should never be

the case in normal running conditions unless the load is very light or the primary leakage induc-

tance is excessive or that an unnecessarily large snubber capacitor is present.

This system avoids the need for an external resistor to program the dead time and contributes with

the multi functional nature of CSD to the IR2161 being realized with only 8 external pins.

Dimming

Almost any Halogen convertor available can be dimmed

by an external phase cut dimmer that operates in trailing

edge mode. This means that at the beginning of the line

voltage half cycle voltage the switch inside the dimmer is

closed and mains voltage is supplied to the convertor al-

lowing the convertor to operate normally. At some point

during the half cycle the switch inside the dimmer is opened

and voltage is no longer applied. The DC bus inside the

convertor almost immediately drops to 0V and the output

is no longer present. In this way bursts of high frequency

output voltage are applied to the lamp. The RMS voltage

across the lamp will naturally vary depending on the

phase angle at which the dimmer switch switches off. In this

way the lamp brightness may easily be varied from zero to maximum output.

Trailing edge dimmers are less common however than

leading edge dimmers. This is because they are more

expensive to make and need to incorporate a pair of

MOSFETs or IGBTs whereas a leading edge dimmer is

based around a single triac.

Conversely many Halogen convertors are not able to op-

erate with leading edge dimmers because of the fact that

they are based around a triac. It is possible however to

design a Halogen convertor that will work effectively with

a triac based dimmer by paying attention to design the

input filter components correctly and to ensure that at the

firing point of the triac the oscillator will start up rapidly. In

the IR2161 based system this is easy to achieve through

the addition of RD and CD which conduct a large current

DC BUS VOLTAGE

LAMP VOLTAGE

Fig. 6 Leading Edge Dimming

Fig. 5 Trailing Edge Dimming

DC BUS VOLTAGE

LAMP VOLTAGE

IRPLHALO1E

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to VCC due to the high dv/dt that occurs when the triac fires and the bus voltage rises rapidly from

zero to the AC line voltage. If the VCC voltage falls below UVLO- during the time when the triac in

the dimmer is off the soft start will not be initiated when voltage because the soft start circuit is not

reset until VCC drops approxmately 2V below UVLO-. This takes some time as the VCC capacitor

discharges very slowly during UVLO micro-power operation. The intermediate period is referred to

as Standby mode.

During dimming the voltage compensation circuit will cause a frequency shift upward at angles

above 90

° because the peak voltage at CS will be reduced, however this will not have a noticeable

effect on the light output.

The problem associated with operation of Halogen convertors with triac dimmers is due to the fact

that after a triac has been fired it will conduct until the current falls below its holding current. If the

load is purely resistive (as in a filament lamp directly connected to the dimmer) this will naturally

happen at the end of the line voltage half cycle as the current has to fall to zero. In a Halogen

convertor it is necessary to place a capacitor and inductor at the AC input to comply with regulations

regarding EM conducted emissions. This means that when the line voltage falls to zero there could

still be some current flowing that is enough to keep the triac switched on and so the next cycle will

follow through and not be phase cut as required. This can happen intermittently resulting in flicker-

ing of the lamps. The way to avoid the problem is to ensure that the product has the smallest

possible filter capacitor CF and to state a minimum load for the convertor. This would be typically

one third of the maximum load to avoid problems of this kind.

EMC Issues

The IRPLHALO1E demo board has not been EMC tested although a filter capacitor CLF and inductor

LF are fitted. The capacitor value may not be increased beyond a certain point to improve filtering

as this causes problems when dimming with triac type phase cut dimmers, i.e. that the phase shift

introduced can prevent the triac current dropping below its holding current at the end of the line

voltage half cycle and so the dimmer no longer functions. In order to prevent this, the proportion of

resistive load presented to the dimmer output must be sufficient for the capacitance. A good rule of

thumb to apply is that no more than 1nF of capacitance should be used per Watt of the maximum

power rating of the convertor, e.g. for a 100W convertor, use a 100nF capacitor. This should allow

dimming to work even at reduced loads without any difficulties. The filter inductor value should be

increased to reduce the conducted emmissions below the limits of the applicable EMC standard.

The inductor should have a powdered Iron core rather than Ferrite as this can handle a much larger

current before saturating.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P17

IRPLHALO1E

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Description:

Power Management IC Development Tools Halogen Cnvrtr 220/230VAC

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