# Low voltage DC power supplies -- how do they make them efficient? **URL:** \ **Category:** Factual Questions\ **Created:** [October 16, 2018, 3:32pm UTC](https://boards.straightdope.com/t/low-voltage-dc-power-supplies-how-do-they-make-them-efficient/823034 "2018-10-16T15:32:51Z")\ **Posts on this page:** 3\ **Page:** 1
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [October 16, 2018, 3:32pm UTC](https://boards.straightdope.com/t/low-voltage-dc-power-supplies-how-do-they-make-them-efficient/823034/1 "2018-10-16T15:32:51Z")
It’s common to make DC power from AC supplies by using a transformer (if voltage change is needed) and a full wave diode bridge, often followed by capacitance and active regulator circuits. But in silicon that bridge drops 1.2 V because of the forward voltage of the diodes. Even with a center tapped transformer and half wave rectification in germanium, there’d be about 0.3 V. If you’re building a power supply for a low voltage, do you just accept that? I’m thinking for example of chargers for individual cells in a large storage system, where you might be delivering a lot of energy at 1.5 or 2 V. The diode drops themselves would make for a 20% to 80% additional loss over the power delivered. Perhaps there are even lower voltage uses for serious DC power? Maybe, I dunno, electroplating or other electrochemical industrial processes? Could they use huge FETs with some appropriate circuit to sense the AC polarity and drive the gates? ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [October 16, 2018, 3:44pm UTC](https://boards.straightdope.com/t/low-voltage-dc-power-supplies-how-do-they-make-them-efficient/823034/2 "2018-10-16T15:44:57Z")
> **[Active rectification](https://en.wikipedia.org/wiki/Active_rectification)** > > Active rectification, or synchronous rectification, is a technique for improving the efficiency of rectification by replacing diodes with actively controlled switches, usually power MOSFETs or power bipolar junction transistors (BJT). Whereas normal semiconductor diodes have a roughly fixed voltage drop of around 0.5-1 volts, active rectifiers behave as resistances, and can have arbitrarily low voltage drop. > Historically, vibrator driven switches or motor-driven commutators have also been used ... Jeez, I should have thought of ways of finding this before I posted… ---
**Author:** ![scr4](https://avatars.discourse-cdn.com/v4/letter/s/59ef9b/32.png) [@scr4](https://boards.straightdope.com/u/scr4)\ **Post date:** [October 16, 2018, 4:19pm UTC](https://boards.straightdope.com/t/low-voltage-dc-power-supplies-how-do-they-make-them-efficient/823034/3 "2018-10-16T16:19:10Z")
Almost everything uses [switched mode power supplies](https://en.wikipedia.org/wiki/Switched-mode_power_supply). And the rectifier is usually upstream of the switching supply. So a 1.2V diode drop is only a 1% inefficiency, regardless of output voltage.