Primary-side leakage

Figure 5-4 How to, and How Not to, Measure Effective Primary Side Leakage Inductance in Flyback...

Measure the in-circuit primary-side leakage inductance Llk in Henries. 

In high-power offline Flybacks, the trace inductances on the secondary side reflect on to the primary side, and can greatly increase the effective primary-side leakage inductance and degrade the efficiency. The situation gets worse when we have to stack several output capacitors in parallel, just to handle the higher RMS currents. Long traces seem inevitable here. This has been discussed in detail previously. 

Ipk is the peak switch current, and Llkp is the primary-side leakage. That certainly is the energy residing in the leakage inductance (at the moment the switch turns OFF), but it is not the entire energy that eventually gets dissipated in the zener clamp on account of the leakage. 

Why did we use the symbol Llk in the dissipation equation above Why didn t we identify it as the primary-side leakage ( Llkp ) The reason is that Llk represents the overall leakage inductance as seen by the switch. So, it is partly Llkp — but it also is influenced by the secondary-side leakage inductance. This is a little hard to visualize, since by definition, the secondary-side leakage inductance is not supposed to be coupled to the primary side (and vice versa). So how could it be affecting anything on the primary side ... 

The zener can therefore see significant dissipation, even assuming zero primary-side leakage. 

In brief, the secondary-side leakage has created much the same effect as a primary-side leakage. 

When both primary- and secondary-side leakages are present, we can find the effective primary-side leakage (as seen by the switch and zener clamp) as... 

So, like any other reactive element, the secondary-side leakage also reflects onto the primary side according to the square of the turns ratio, where it adds up in series with any primary-side leakage present. 

Measuring the Effective Primary-side Leakage Inductance... 

You can argue—but 20nH per inch is just a rule of thumb How can we even confirm what the effective leakage inductance really increases on the primary side (as a result of that) ... 

Secondary-side Leakages also Affect the Primary Side... 

The magnetization current component is not coupled by transformer action to the secondary. In that sense, it is like a parallel leakage inductance. We need to subtract this component from the total switch current, and only then will we find that the primary and secondary currents scale according to the turns ratio. In other words, the magnetization current does not scale — it stays confined to the primary side. 

The leakage inductance of both coils has been modeled by an inductor in series with the load, since the current in the coils also produces leakage flux. These inductances are labeled L p and Lj, respectively. Notice that the leakage inductance for the secondary side has been divided by the turns ratio n, squared because it was reflected to the primary side. Resistors labeled Rp and Rj have also been placed in series with the load to represent the resistance of the conductors used to wind the coils. Again, the secondary resistance has been divided by the square of the turns ratio, since it was reflected. 

The nonideal characteristics of a transformer include core and winding losses, presence of leakage fluxes, and finite permeability of the core. Hence, the actual model should include the physical representations of these nonideal characteristics. This is shown in Fig. 10.105(b), where the shunt magnetization and core loss components have been ignored for the sake of convenience. Such approximations are common in transformer analysis and only cause trivial inaccuracies in computation. Figure 10.106 shows the equivalent circuit of Fig. 10.105(b) with the secondary-side impedance referred to the primary side. 

After fabrication, the units were subjected to 50 primary-side thermal cycles covering temperature changes more severe than those likely to be encountered in subsequent operation. The units were then helium-leak-tested at atmospheric pressure with mass-spectrometer equipment capable of detecting leakage lower than 0.1 cc of helium at STP per day. Leaks were repaired and the thermal-cycle test and leak test w ere repeated until no leakage w as detectable. 

The turboexpander dry gas seal consists of the conventional dry gas seal mating ring and primary ring, an outboard labyrinth, an inboard labyrinth, and tlie cavity to be vented, if desired. Tlie outboard labyrinth reduces warm seal gas leakage to the process side efficiency deterioration is thus minimized. The inboard labyrinth, on one hand, provides an additional seal between the process and lubricating fluids. On the other hand, it allows injection of an inert gas, if desired. In the latter case, inert gas leaks to the bearing side and to the cavity between the... 

For the tandem arrangement gas seal, a primary seal vent must be pro vided to vent the leakage across the process side seal. This vent ma> lie to flare or other suitable gas disposal point. The back pressure under nor mal conditions should be kept to a low value. A small amount of back pressure is recommended to keep a positive differential across the see ondaiy seal. Leakage measurement may be provided in the vent line to provide health monitoring of the primary seal. Unfortunately, the rotameter, which would be the obvious choice, should not be used because of its lack o reliability. If an orifice or needle valve is used to set the back pressure to the seal vent, pressure upstream of the restriction can be measured for a relative flow measurement. This type of reading does provide trend data that may be used to judge the seal s performance. 

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