# A question comparing DC-DC converter and transformer

Transformer is a passive component where the input power is equal to the output power. This comes from the conservation of energy hence Pin = Pout.

Can we say the same thing for a DC-DC converter that Pout cannot be more than Pin? Let's say we have a boost converter and we only know its input current, input voltage and output voltage. If it is 5V to 12V converter with 100mA input current can we say that the theoretical max output current is (5V*100mA)/12V = 41mA?

• In theory yes if it were 100% efficient and didn’t store much energy like a transformer, but no , the LC parts can store 10x the load on startup unless a soft-start is used. Jun 1, 2021 at 11:46
• Oh so output power can be bigger than the input power since it stores energy? Jun 1, 2021 at 12:09
• No output power while input is being ramped up. This also means very low input impedance on startup, but in steady state voltage and Impedance ratios applied if 100% eff. Jun 1, 2021 at 12:12

The energy out won't ever be more than the energy in.

You can have a high instantaneous output power if the DC-DC converter has energy storage, as long as the long term continuous output power is lower than the input power.

Consider a flash-gun. It charges the storage capacitor over a few seconds, perhaps pulling a watt from the small batteries. For the fraction of a millisecond that it's driving the flash tube, it could be delivering kilowatts.

But it won't ever deliver more energy than it can draw from the batteries.

For steady state conditions, the output power cannot exceed the input power so your calculation is correct. However the principle of conservation of energy (not power) allows for the output power to exceed the input power for a finite time. Thus you can store energy in a capacitor and then release it quickly so the output power can be high but only for a short time. This technique is used in ignition systems for example.

Yes that is correct. Due to conservation of energy, $$\P_{in}\$$ must always be greater than $$\P_{out}\$$. And, on an ideal converter, they will be the same.

Of course, this might seem strange at first, specially on a DC-DC converter that lowers the voltage (buck converter, i.e.), because the current might be greater in the output.

Nonetheless, electrical circuits have perfect power balance. Meaning that the sum of all power consumptions must be equal to all power generations. If your analysis follows Kirchhoff's laws preciselly, this outcome is inevitable.