X-capacitors are part of the EMC input filter of power supplies (see diagram right), that are essentially connected directly to the appliance’s mains plug and, uncontrolled, are left with up to the peak ac voltage (~340V) when an appliance is unplugged.
This means that the disconnected plug could discharge this capacitor into anyone touching its pins, which is why mechanisms to discharge X-capacitors are mandated.
A simple resistor can do the job, but results in constant power wastage all the time the appliance is powered.
To save this power, Power Integrations came up with a family of chips called CapZero.
They are two terminal devices that are essentially open circuit all the time an AC voltage is applied, but switch to short-circuit when the ac signal is removed. In conjunction with a series resistance, this allows them to empty X-capacitors once an appliance is unplugged.
CAP300DG, 54kΩ total resistance, 6µF X-capacitor, 230Vac
The latest generation, CapZero-3 (link will be live on 20 August), has the same 1kV and 6µF max rating as the earlier CapZero-2 family, but has increased discharge current capability, allowing it to work with series resistance down to 54kΩ (CapZero-2 was 142kΩ) and up to 6µF with 230Vac inputs.
This allows it to be used to meet the IEC60335 appliance safety standard . “To protect the user from an electrical hazard, it requires the voltage across the input X capacitor to discharge to less than 34 V within less than one second after the AC is removed,” according to the firm, which added: “CAPZero-3 ICs permit the use of larger X capacitors, which in turn enables smaller inductive components to be used with no resulting change in power consumption.”
The chip allows IEC60335 and IEC62368 standards to be met, according to the firm, and both Nemko and CB certifications are in process – with full certification expected in November – so developers will not need to perform a separate safety test on the X-capacitor discharge circuit of the power supply.
While 54kΩ connected straight across the capacitor would have a similar discharge effect, it would have a constant dissipation of 1W when powered. Using the new chip, Power Integrations claims this will be reduced to <4mW (exact figures will be revealed tomorrow).
The chip is actually rated for any capacitance value from 100nF to 6µF, and can be placed before or after a system’s input fuse.
On the 1kV device rating, PI spokesman Peter Rogerson said: “there will be no line surge issues where ever you are on the planet.”
He claims high reliability for the parts, with a part failure rate of 0.1ppm. “If you are building an expensive appliance, you are not going to have to pull it out of the wall because our part has failed,” he claimed.
Creepage on the package (plastic SO-8) and PCB is maintained at >4mm.