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1Clear out junk files and repair common Windows errors2Scan for outdated or missing drivers - takes under a minute3Repair Windows errors before they cause bigger problemsA switched X-capacitor discharge circuit can reduce the power a mains-powered supply wastes in its bleeder resistor: it disconnects the resistor while AC is present, then reconnects it after AC is removed so the capacitor can discharge. The tradeoff is that the circuit must still meet the applicable discharge and safety requirements for the finished product.
Why an X-capacitor bleeder uses power
An X capacitor sits across line and neutral in an AC-DC supply’s electromagnetic-interference (EMI) filter. It helps suppress differential-mode noise, but it can retain a charge after the product is unplugged. A resistor placed in parallel with the capacitor provides a path for that charge to dissipate.
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With a passive bleeder, the resistor also draws current whenever mains voltage is applied. Its approximate continuous dissipation is P = V²/R, where V is the AC RMS voltage across it and R is its resistance. For the same resistor, higher line voltage means greater loss; reducing resistance to discharge the capacitor faster also increases normal-operation loss. Texas Instruments explains this mechanism in its application report.
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How a switched discharge circuit works
Instead of leaving the bleeder resistor permanently across the capacitor, a switched design puts an electronic switch in series with the resistor path. While AC is applied, the switch interrupts that path and avoids most of the resistor’s continuous mains-frequency loss. When AC is removed, the circuit detects the change and reconnects the resistor so stored charge can drain away.
This approach separates two jobs that a passive resistor performs at once: limiting residual charge after unplugging, and drawing current during normal operation. A switched circuit can reduce the latter without eliminating the former. It does not, by itself, establish that the complete power supply meets its safety, EMI, or standby-power requirements.
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How much loss can it avoid?
The possible reduction depends on the bleeder loss the design would otherwise have, plus the switched circuit’s own connected-state consumption. Manufacturer calculations and specifications use different voltages, capacitances, and methods, so their figures should not be treated as a head-to-head ranking.
| Example | Published figure | How to interpret it |
|---|---|---|
| 1 MΩ passive bleeder at 230 V RMS | About 53 mW, calculated by Power Integrations in a 2016 application note | Illustrative resistor loss, not a universal supply measurement. |
| 1 MΩ passive bleeder | 14.4 mW at 120 V RMS and 52.9 mW at 230 V RMS, calculated by the Power Sources Manufacturers Association in Q1 2020 | Examples show how line voltage changes dissipation for the same resistance. |
| Power Integrations CAPZero family | Less than 5 mW at 230 VAC, according to the current product page accessed 2026-10-07 | Manufacturer-stated consumption; not an independent measurement or a CAPZero-4-specific specification. |
| Power Integrations CAPZero-4 | Below 0.75 mW at 230 VAC, according to the company announcement dated 2026-09-15 | Manufacturer claim for this generation. The same announcement claims up to 1000 mW system no-load/standby input-power reduction; that is not a guaranteed saving in every supply. |
| NXP TEA1708T | 1 mW typical at 230 V AC; 2.3 mA maximum discharge current, according to the product page | Manufacturer specifications, not a direct comparison with the other entries. |
| MPS HF81 | At 265 VAC, datasheet calculations state savings of 86 mW at 1 μF, 191 mW at 2 μF, and 464 mW at 5 μF | Calculated examples in a 2015 datasheet revision, not measured universal savings. |
These numbers are not interchangeable. Voltage, total X capacitance, resistor values, operating state, and calculation or measurement method all affect the result. A 200 W supply using larger X capacitors can have discharge-resistor losses around 125 mW, according to Power Integrations’ 2016 application note; this is an example, not a typical value for every 200 W supply.
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Examples of discharge ICs
Several manufacturers document this general switch-and-bleeder approach. The examples below are not endorsements, and their specifications are not a substitute for checking the exact device documentation and lifecycle status.
- Power Integrations CAPZero-4: The company announced on 2026-09-15 that the part was in production and shipping, with a stated application range of 100 nF to 6 μF line-to-neutral capacitance. Its below-0.75-mW-at-230-VAC figure is a manufacturer claim. See the CAPZero-4 announcement. The CAPZero product page gives family-level information and says CAPZero-2 is not recommended for new designs, with CAPZero-3 suggested as an alternative; check generation-specific documents rather than applying family-page claims to every part.
- NXP TEA1708T: NXP lists 1 mW typical consumption at 230 V AC, a maximum discharge current of 2.3 mA, and an externally set discharge delay. Consult the TEA1708T product page and linked datasheet for design details.
- MPS HF81: Its cited datasheet describes a switched bleeder topology and provides calculated examples. The cited revision is dated 2015-09-28, so verify current lifecycle and availability before considering it for a new design. See the HF81 datasheet.
Power Integrations also reports that more than 1 billion CAPZero ICs have shipped since the technology’s introduction in 2010. This is a company-reported shipment figure, not a performance comparison.
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What to check before choosing a circuit
A low connected-state consumption figure is only one part of the design decision. Check the complete circuit against the supply’s electrical, thermal, safety, and sourcing constraints:
- Capacitance and discharge target: Account for total line-to-neutral X capacitance and confirm the required discharge time and residual voltage for the product.
- Mains and surges: Confirm the device and external components suit the product’s voltage range and surge environment.
- Resistor selection: Size discharge resistors for resistance, power and voltage ratings, temperature, and fault behavior. The switched IC does not remove the need to design this path.
- EMI filter: Confirm that the X-capacitor value still meets EMI needs and assess whether changing it affects other filter components.
- Device implementation: Check temperature range, package and creepage requirements, safety approvals relevant to the finished product, board area, and whether discharge is integrated into the main PWM controller.
- System power budget: Measure or calculate the complete supply’s connected-state input power under the relevant conditions. An IC’s consumption figure is not the same as the finished product’s total standby power.
- Lifecycle and sourcing: Verify current documentation, lifecycle status, and supply before committing a design.
Safety requirements depend on the product and standard
The reason for discharging the capacitor is to limit the charge that remains accessible after disconnection. Published summaries do not state one universal timing rule: TI’s 2019 report cites IEC 62368 clause 5.5.2.3 as requiring a time constant no greater than two seconds; the PSMA’s 2020 overview describes below 34 V peak in less than one second for IEC 60335 and IEC 62368; and Power Integrations’ product page gives under one second as its design guidance for a worst-case RC time constant.
These statements should not be collapsed into a single compliance threshold. The applicable normative standard, edition, test conditions, geography, and product class determine what a particular design must satisfy. Confirm requirements against the standard itself and the relevant certification review; a discharge IC alone does not certify the power supply.
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