Any reason I cannot just switch a transformer from a 555 to generate high voltages?

Question

I had an idea to finally start using my nixie tubes and thought why not just switch a mosfet which switches a transformer to generate required high voltages for nixie tubes from a low voltage supply. When I googled all I found was these boost converters using inductors. Now is there a reason for these being used?

Answer 1

Both boost converters and transformers can give higher voltage from lower one. Here are some specifications:

Boost converters:

• Require less space than transformers
• Require more components to work
• Light

Transformers:

• Require more space
• Require less components to work
• Heavy

According to this, if you want to use less space, use boost converter.
If you can afford more space, then use transformer.

Answer 2

What you're describing (switching a transformer at high frequency via a transistor) is called a forward converter. These are more difficult to design than a boost converter. In particular, you have to limit the duty cycle to keep the current in the windings stable.

Transformer-based converters are used to isolate a system from the mains power. If you already have an isolated DC supply, it's much simpler and safer to use a boost converter with an off-the-shelf controller. Trying to build your own switch-mode power supply from scratch is a bad idea unless you really know what you're doing.

Alternately, if you can use mains power you could transform that to the ~200V needed by the tubes and rectify it without needing any DC-DC conversion.

Answer 3

Yes, of course there is a reason boost converters are used to make high voltage from a low voltage supply. The reason is that's what they do, or put another way they meet the spec.

A boost converter with a single inductor is the simplest way to get a moderate boost in voltage with significant current, it takes only a single low-side switching element, and can be easily controlled via PWM. For high stepup ratios, especially when significant current is required, a transformer starts becoming worth the extra complexity. At low currents there are cascaded charge pumps, but those require one stage per multiple. For example, to get 10x higher voltage requires 10 stages ideally, more in practise due to some voltage loss per stage. Look up Cockroft-Walton for more details on a particular cascaded charge pump.

Old televisions used a flyback transformer topology to make 10s of kV from a few 100 V DC. The 10s of kV were needed to drive the electron gun in the picture tube.

There are lots of ways of making a higher voltage from a lower one. Which way is "best" depends on the particular circumstance and on what criteria are most important to you. There is no single universal Right Answer.

Answer 4

Boost converters using a single inductor are the most common type and they also accommadate the smallest amount of power transfer capabilities i.e. they are in the below-1 watt to 10 watt range loosely speaking. A small disadvantages is that they provide no isolation between input voltage and output voltage AND, the peak ratio of step-up voltage to supply voltage is limited probably to less than 100:1 although there will be notable exceptions.

The transformer flyback circuit is often used in AC-DC converters i.e. the wall wart and these range in power from below 1 watt to well over 40 watts. They can be step-up types of course.

Raising the bar a bit is the half bridge (push-pull) transformer design. This uses a centre tapped primary and two (usually) MOSFETs pushing and pulling the primary. I guess these tend to cover the range 25 watts to over 200 watts.

The main circuit of higher powers is the H bridge and these are responsible for AC inverters etc..

So, if you want to know which topology to choose concentrate on power requirements and whether isolation is needed. All apart from the boost (single inductor) circuit can provide isolation.