A switched X-capacitor discharge circuit can reduce a power supply’s connected-state loss by disconnecting the discharge resistor while AC is present, then reconnecting it after AC is removed. The resistor still provides a path for stored charge to dissipate, but it does not draw current continuously during normal operation. Whether a design is safe and compliant depends on the complete circuit and the applicable product standard—not on the discharge IC alone.
Why an X-capacitor bleeder consumes power
An X capacitor sits across line and neutral in an AC power supply’s EMI filter, where it helps suppress differential-mode conducted noise. It can retain charge after the product is unplugged, so the design needs a means to discharge it.
A passive bleeder resistor connected in parallel with the capacitor provides that path. But while the supply remains connected to mains, the resistor is also exposed to the AC voltage and dissipates power. Its approximate loss is P = V²/R, using the AC RMS voltage for V. For the same resistor, raising the line voltage increases loss quadratically; reducing resistance to discharge the capacitor faster increases loss as well.
Power Integrations’ 2016 application note calculates about 53 mW for a 1 MΩ resistor at 230 V RMS. The Power Sources Manufacturers Association’s Q1 2020 overview gives calculated examples of 14.4 mW at 120 V RMS and 52.9 mW at 230 V RMS for a 1 MΩ resistor. Power Integrations also cited around 125 mW of discharge-resistor loss in a 200 W supply using larger X capacitors. These figures use different examples and should not be treated as directly comparable without matching capacitance, resistor values, voltage, and calculation conditions. Texas Instruments’ 2019 application report explains the standby-loss mechanism and the V²/R relationship; PSMA’s Q1 2020 overview provides its examples.
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How a switched discharge circuit works
An active discharge circuit places an electronic switch in series with the discharge resistor or resistor network. While AC is present, the switch interrupts the resistor path, reducing the bleeder’s connected-state loss. When AC is removed, the circuit detects the change and reconnects the discharge path so the X capacitor can lose its stored charge.
This separates two needs that conflict in a passive design: keeping a discharge path available after unplugging, and avoiding continuous resistor current while the supply is operating. The switch does not eliminate the resistor or the need to size it correctly. Its operation, the resistor network, the X capacitor, and the rest of the power supply must work together.
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Examples of discharge ICs and published claims
The following figures come from manufacturers or a datasheet, not from an independent, side-by-side laboratory comparison. Different capacitance ranges, conditions, and measurement methods mean the numbers are not a ranking of devices.
| Example | Published information | Source and qualification |
|---|---|---|
| CAPZero family | Less than 5 mW at 230 VAC; the product page also gives design guidance for a worst-case RC time constant below one second. It says CAPZero-2 is not recommended for new designs and suggests CAPZero-3 as an alternative. | Power Integrations’ current CAPZero product page; manufacturer statements, not independent measurements. |
| CAPZero-4 | Below 0.75 mW at 230 VAC and a stated application range of 100 nF to 6 μF line-to-neutral capacitance. Power Integrations also claims up to 1000 mW of system no-load/standby input-power reduction; that is not a guaranteed saving in every supply. | Power Integrations’ announcement dated September 15, 2026, which says the product was in production and shipping at that time. The company also reported more than 1 billion CAPZero ICs shipped since the technology’s 2010 introduction. These are vendor statements. Announcement. |
| NXP TEA1708T | Typical consumption of 1 mW at 230 V AC; maximum discharge current of 2.3 mA; discharge delay is set externally. | NXP’s TEA1708T product page; its linked datasheet is Rev. 1.1, dated April 6, 2020. |
| MPS HF81 | Datasheet table calculations at 265 VAC show savings of 86 mW for 1 μF, 191 mW for 2 μF, and 464 mW for 5 μF. | MPS HF81 datasheet, Rev. 1.14, dated September 28, 2015. These are datasheet calculations, not measured universal savings; verify lifecycle and availability before considering it for a new design. |
The product page’s less-than-5-mW figure applies to the cited CAPZero family page; it is not the CAPZero-4 specification. CAPZero-4’s separate below-0.75-mW figure is a manufacturer claim. For any candidate, check the documentation for the exact generation and part number rather than transferring a family-level or earlier-generation figure to another device.
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How to decide whether an active bleeder fits
Start with the actual supply, not a headline power figure. The relevant comparison is the passive resistor loss the design would otherwise incur against the active circuit’s connected-state consumption under the same mains and operating conditions. The reduction in input power for the whole supply may differ from the IC’s own published consumption, and the sources cited here do not establish a universal whole-supply saving.
- Capacitance and discharge target: Establish the total line-to-neutral X capacitance and the required time and residual voltage after disconnection. Confirm the candidate device’s supported capacitance range and discharge behavior.
- Mains and surges: Check the intended input-voltage range and surge environment, as well as the switch and resistor network’s voltage and fault ratings.
- Resistor network: Select resistor values, power ratings, and voltage ratings for the discharge requirement and fault conditions. A switched IC does not make undersized or unsuitable resistors safe.
- EMI-filter design: Confirm that the X capacitor still delivers the needed noise suppression. Changing its value can affect other filter components and overall EMI performance.
- Thermal, mechanical, and sourcing constraints: Review the IC’s temperature range, package and creepage requirements, safety approvals relevant to the final design, board area, cost, and lifecycle. Also check whether discharge is already integrated into the main PWM controller.
Discharge timing and standards require product-specific verification
Technical sources describe discharge limits differently, so do not combine them into a single universal rule. TI’s August 2019 report cites IEC 62368 clause 5.5.2.3 as requiring a time constant no greater than two seconds. PSMA’s Q1 2020 article states below 34 V peak in less than one second for IEC 60335 and IEC 62368. Power Integrations’ product page gives a worst-case RC time constant below one second as design guidance for its design.
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Those statements are not a substitute for checking the applicable normative standard. The governing requirement depends on the product class, standard and edition, test conditions, and relevant geography. Verify it against the standard text and certification review for the finished product; an IC’s presence or a vendor’s application guidance does not certify the complete power supply.
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