The Specific Design Problem

A project log for 350 Watt True Sine Inverter

Functional design revision of the prototype TS350.

Brian CornellBrian Cornell 05/09/2019 at 19:530 Comments

I did a poor job on the systems engineering for this project.  I spent a lot of time on specific problems like the layout of the inverter bridge, rectifier switching losses, heat sinks, firmware, etc. but I was too myopic.  I didn’t bring it all together early enough in the design and this resulted in two major deficiencies.

Thermal management.  I spent much time modeling heatsinks’ thermal performance, tab insulator materials, mounting height considerations to minimize parasitics and - key - the coefficients of each transistor on the dual sinks.  But I stopped there:  I compartmentalized the thermal models and did not establish the coefficients of adjacent areas or consider the entire system given hot air would be circulating in an enclosure.

I should have mocked-up the full layout with all key heat sources to establish coefficients and determine the size fans it would need.  The as-built dissipates ~ 68 watts of heat at full load.  Had I better established this early on I would have changed fan positions and sizing or maybe the entire design.  I was fortunate that I had a convenient spot, and the power budget on the auxiliary supply, to add a third fan.

An early design goal was, aside from board size, for the unit to be enclosure agnostic.  That’s how the pull/push arrangement for the fans on the board came to be.  I realize now that in a design dealing with this much dissipation that the enclosure and the gear going in it are inextricably linked.  An ah-ha related to this is that a fan must be positioned at the enclosure to ambient interface to ensure air is forced in or out of the unit.  Otherwise internal fans will only be moving hot air around.

I also might have thought twice about trying to get this much power out of it.  A two layer, two ounce PCB is woefully inadequate to handle this much current.  Traces turn into copper pours, grabbing more space and reducing layout efficiency.  Sometimes both layers must be used wasting even more.  In spite of a very generous layout for the inverter primary it is dissipating almost five watts.  That might not sound like much against the total losses but it is concentrated around other high dissipators and contributes significant heat stress.

AC Voltage Regulation.  The sine controller has eight DDS sine patterns stored in flash and selects one based on the transformer’s auxiliary supply voltage (a proxy for the HV supply).  It does a good job of keeping the no / light load voltage between 115 - 120V AC RMS.  Under heavy loads the output tends to droop towards 100V.  Most electronic devices won’t care but small motorized appliances (like a Dremel or mixer) will.

A few things contribute to this.  Transformer regulation is poor; but this can be compensated for by adding a few turns to the secondary.  Second, the DDS pattern imposes a voltage margin on the HV supply (you can see this in the Excel model).  Essentially, the output voltage from the sine pattern on any given cycle is:

    Vhv * Tdc = Vout

    Where Vhv=High voltage supply
    Tdc = Duty Cycle, percent
    Vout = output voltage

The sine bridge on times are highest at the sine peak with a low input voltage.  The DDS patterns are designed to ensure that there is a discernible off / on time for the same side high / low pair respectively to avoid high switching losses and maintain bootstrap capacitor charge.  It also reduces the chance of violating transistor SOA and a resulting failure should a short or transient occur at just the right time.